Raffaele-

Strange, all 3 formats (m->m, m->s, and s->s) should be part of the .aaxplugin that was installed.

-Does the track you're trying to place it on have the ability to be a stereo track? (If you're routing the mono track to another mono track, or trying to place UltraReverb before another plug-in in the signal chain which is mono, Pro Tools won't let you do mono-stereo).

-Can you try running the installer again if the above question isn't an issue?

Best,

Tom