You can use a combination of the "Path to CSS file" and "CSS Selector" configuration parameters to get the desired result. Although instructions are there within the manager for the CSS Selector option, here’s how they should be entered:
displayName=selectorName;displayName2=selectorName2
For instance, say you have .mono and .smallText selectors in your CSS file, you could add them here as:
Monospaced text=mono;Small text=smallText
Note that the last entry should not have a semi-colon after it.
For the "Path to CSS file" option, create a simple CSS file of nothing but classes and such that will coincide with the classes you’re using for your Styles dropdown. This will let editor see how content might look when it’s published. You might need to tweak the CSS file after the fact to get the desired results since TinyMCE will likely display the CSS a bit differently.
Hope this helps!
Jeff