In large milling machines edge finding is done using a special tool called, you guessed it, an Edge Finder! Unfortunately it's pretty rare to find an edge finder that will fit in the Othermill. See the wikipedia article on Edge finders: http://en.wikipedia.org/wiki/Wiggler_%28tool%29
While Otherplan has locating feature for getting pretty close it's very hard to much more accurate than a few thousandths of an inch. I still use the built-in edge finding for locating a new rough material within the mill.
Step 1: Find your tools!
A good dry erase marker
A bit with a precise 1/8" shank, a broken bit is fine, as long as the shank is in good shape. I would recommend a bit you don't care too much about, e.g. a carbide bit is way too easy to break when mounting as a probe.
Optional: a caliper or external micrometer to verify the 1/8th tool shank.
A mill of some kind, but I expect if you have a mill other than the Othermill, you likely already have an edge finder ;)