Amber -
It should be in the list once you select "Numeric" if it's not you might check a few things: 1. that you have the right permissions to create it, I've found sometimes that E10 lets it look like you created something and then won't save it if you've run out of room or don't have create credentials 2. if that is all correct you might check with support. The other thing I didn't think about but might be holding you back - if you're trying to modify a field that is system standard with a new format that might not be allowed either.
Best,
Lauren