Thanks for the reply. I added them via the tool in ArcCatalog. I'm pretty sure the Direction field was 12 characters; I tried changing the length but can't do that on an existing field afaik. I did enter the direction in the format N 45.2367 E rather than N 45-23-67 E, but either way they should fit in field length 12. I can enter the segments, and they display properly, until I click Finish.
I didn't used to have this problem; maybe it's related to one of Arc 10's SPs?