This is a fun question..
Most people on this forum will tell you that you need to create two different overlapping text boxes, one with a bold variant of the text and the other with a plain variant, and then create scripts that will clear the content of either box depending on a field value.
But there is also a much less roundabout undocumented way to accomplish this that relies on the fact that PSM recognizes RTF tags:
IF (MOD(COUNTER(), 2) = 1, "{\rtf1\b This is bold\b0}", "This is not bold")
Of course the test part of this condition is just an example, you can base it on a field value instead. The \b tag switches to bold (it needs to be followed by a space) and \b0 resets to non-bold.
Just be aware that you're not allowed to mix RTF content and plain text content in the result of a single variable, so if you want to do that you'll need to separate the text out to different variables.