Hi silvine,
First, be sure you’re running the latest version (2.0.3), which changes some of the full screen behavior. With that, you can switch applications without hiding Scrivener, so if you call up the dictionary app while in FS it will appear overtop your FS space. It will vanish again when you click back into Scrivener to write, however, so you’d need to use cmd-tab (or any other method) to bring the dictionary back when you wanted to use it again. There’s also the nice little ctrl-cmd-D trick which brings up a little dictionary window to define the selected word (you can switch to thesaurus as well or use the “More…” link to open the full app).
If you really want the dictionary to remain visible while writing in FS, you could use something like Afloat to keep it floating atop your full screen. Alternatively, you could use Ioa’s developer dashboard widget trick to keep the Dictionary widget floating:
Hope that helps!