Hi Tomas,
Don't worry, the support systems are perhaps not as intuitive as they could be (though hitting F1 in the pattern editor should have given some clues). To play the current pattern hit F6 or F7 (to play from the cursor), to play the song press F5 or the play button in the host. Note, however, that you have to have defined a song first - using the order list (F11). This is simply a list of the patterns you want to play, and the order you want to play them in. If you hit F5 in v0.85.1, you should get a message warning you if the order list is empty.
reViSiT doesn't use MIDI to communicate with the host - most VST hosts pipe sync info more directly to the plug-ins that request it - such as, sample position, tempo, etc.
Good Luck!
All the best,
Chris
PS: A play button might well be a feature of a future version