Odd thing: I can't get 100%. Trying to set it to 100% (even in the
Percentage: box) results in 106% for me. So I'll assume 106% == 100%.
There's some keyboard shortcuts listed in the help document here:
Zoom in: AltF6
Zoom out: AltShiftF6
Fit to window: CtrlShiftW
They didn't document a zoom to 100% (actual size).
Noticing that CtrlShift turns my cursor into a magnifying glass, I started hitting random keys.
Zoom to 106% (100%? See below.): CtrlShiftI
Second odd thing: Ctrl+ and Ctrl- do absolutely nothing for me. In Firefox they zoom, with Ctrl0 being restore to normal size, but they do nothing for me in Visio 2010.
Apparently the 106% thing is a known bug related to the metric drawing templates. There's a workaround at that link, but you should be fine if you're using US templates.