I assume you did a multiple regression analysis (the word "test" here is a strange use of that term). If so, the first thing I would say is that your residuals do not look that worrisome. Multiple regression assumptions are very forgiving - they are rarely satisfied but it usually doesn't matter much. However, to the extent that your residuals are skewed, it is natural to try a log transformation. The easiest way to do that is when you put your Y variable in the box for the response variable, click on the drop down next to "Transform" in that window and ask for the log. Then use the same Model Effects that you were using.
My guess is that the log transform may not improve things much here since the residuals had a negative skew (I think they would be more appropriate for a positive skew). So, you could try some of the other transformations. However, as I said, I don't think your residuals look that much in need of modifying the model. I'd look at the other aspects of your model (are the coefficients for the effects significant and do they make sense? is the overall model fit good? what other data do you have that you did not use? what does the residual plot look like - is the scatter reasonably random?) before worrying too much about the distribution of the residuals.
