Ahh that's just simple. You hold the two mouse Buttons down while the machine powers up.
I would use that 15kHz monitor because I'm not sure about your Sun monitor, but I have a Sun GDM-20e20 (yeah it's a Sony rebadged Trinton) and it doesn't like syncing down that low.
Just watch to see if any of the diagnostic colors are popping up too
Light Grey, you test passed
Red, You have a ROM error or something probably got loose
Green, Chip Ram error, those SIMMS like to work themselves loose sometimes
Blue, Custome Chip error, just resocket them
Yahoo, some kinda software error, or other things two
Yeah, make sure you are booting in a friendly resolution 640x200 should be nice when you are installing that Cybervision 64.