I cannot add a student to the conference registration.

This can be caused by several things.  

1) Make sure all students have a gender and t-shirt size set.

2) Verify that they are on your National Roster.

3) Verify that you have submitted your National Roster (this is a different process then just uploading/adding students to the National Roster)

4) Check to see if that student has a green check under the "Verified" column.  If you see a red X, then they are not verified via the National Roster and #3 above is probably the reason.