Hey David,
Right now, you cannot enter coordinates directly.
In v1.2, you will be able to enter coordinates in a variety of formats to add markers to the map. We're pushing that update to Apple tomorrow or Monday, so you should get it in two weeks.
In v1.3, we'll add a feature to guide you to the waypoints you create, and you should get this hopefully by late October. We would have liked to include that in v1.2, but we want to get the other improvements we have already made out to people immediately.
Kind regards,
Andrew
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Andrew Johnson
CEO, TrailBehind.com
415-336-1379