Testing for moderation with continuous vs. categorical moderators I am testing an interaction effect where $X$ and $Y$ are continuous variable and $M$ (Moderator) is a categorical variable (effects coding $+1$, $-1$). 
I have no clue about how to do a post-hoc probing of slopes in this case.
For a continuous moderator we calculate $Z_{\rm above}$ and $Z_{\rm low}$, calculate their crossproduct with $X$ (e.g., $Z_{\rm Above}*X$ and $Z_{\rm low}*X$) and then run the regression with their combinations of ${\rm High}$ and ${\rm Low}$ with $Y$.
I do not see that the same method above could be applied for a categorical moderator. How can I test that?
 A: I'm not sure I follow your strategy with continuous moderators, but the proper approach with categorical moderators is essentially the same as with continuous ones.  Effect coding is a perfectly reasonable strategy for representing categorical covariates, and it makes no difference in terms of how you test the interaction.  To test an interaction, we add a product term to the model, and test the beta associated with that term.  Assume $Y$ is normally distributed, so that we're talking about a standard linear regression model, and that $X$ and $Z$ are continuous.  Let's say that you are primarily interested in the relationship between $X$ and $Y$, but think the specific nature of that relationship may depend on the value of $Z$, so that we will call $Z$ a moderator here.  Then, the model is:
$$
\hat{Y}=\hat{\beta}_0+\hat{\beta}_1X+\hat{\beta}_2Z+\hat{\beta}_3XZ
$$
The test of $\hat{\beta}_3$ will assess the existence of the interaction.  Note that you have to include $Z$ in the model (see here and here for discussions of that issue).  Now, let's consider a situation like yours where you wonder if a categorical covariate, $Z$, with two levels moderates the relationship between $X$ and $Y$, then your model would be:
$$
\hat{Y}=\hat{\beta}_0+\hat{\beta}_1X+\hat{\beta}_2Z+\hat{\beta}_3XZ
$$
exactly the same! Note that the coding strategy for $Z$ is not represented--it's irrelevant, reference cell coding or any other valid coding scheme would be employed and tested the same way.  Again, you examine the test of $\hat{\beta}_3$ to see if the moderation is 'significant'.  
The situation is a little more complicated if your categorical covariate has more than two levels.  As you probably know, categorical covariates with $k$ levels are represented by $k-1$ 'dummy' variables.  Thus, for example, if $k=3$, you need two new variables.  Let's assume the situation is as above, but you are wondering if the relationship is moderated by $Z$, a categorical covariate with an arbitrarily large number of levels.  Then the model would be:
$$
\hat{Y}=\hat{\beta}_0+\hat{\beta}_1X+\hat{\beta}_2Z_1+\hat{\beta}_3Z_2+\cdots+\hat{\beta}_kZ_{k-1}+\hat{\beta}_{k+1}XZ_1+\hat{\beta}_{k+2}XZ_2+\cdots+\hat{\beta}_{2k-1}XZ_{k-1}
$$
That's an ugly formula, but it's the way it's done.  The important part is this: because you now have more than one $Z$ variable, to test if the moderation is 'significant', you drop all $k-1$ interaction terms, fit the reduced model, and perform a nested model test.  In a standard linear regression context like the situation we're assuming here, that can be the F change test:
$$
F_{change}=\frac{\left(\frac{SSE_{reduced}-SSE_{full}}{k-1}\right)}{\left(\frac{SSE_{full}}{df_{full}}\right)}
$$
where $SSE$ is the sum of squared errors from the ANOVA table, and $p$ is the number of parameters (betas) you are estimating for that model.  This $F_{change}$ value is assessed against the $F$ distribution with $(k-1,df\ {\rm error}_{full})$ degrees of freedom.  If you were working with the generalized linear model, you would use the likelihood ratio test instead.  (NB, most software can do these tests for you; e.g., in R the anova() command can perform nested model tests.)  
To understand the effect of the moderation, I think it's best to make a scatterplot of the data and superimpose several regression lines over the points, one for each level of the moderator (i.e., $k$ lines, not $k-1$).  In addition, it's typically best to plot the points associated with the different levels of $Z$ with different symbols and colors.  If your moderator is continuous, it's often convenient to plot lines at the mean of $Z$, 1 SD above the mean and 1 SD below, which is what I think you are referring to in the question.
