To use any voicebank, open first an empty UST file.
Then, select the folder of the voicebank (that contains wav files, readme.txt and oto file) that you want to use and just drag and drop that into UTAU-Synth window.
After that, it should ask if you want to use that VB, click right option この音源に切り替える(means "change to this voicebank").
Now the whatever voicebank what you want to use should be inside UTAU-Synth.