Hi Everyone - I just wanted to post this because I fell into the same trap and wanted to make sure others can find the solution JIC.

If you are modifying a custom registration form (very cool piece mind you) its easier to use the controls in Kentico.

1) Navigate to System tables - find the user table, go to alternative forms
2) Navigate to Layout (after you've added your fields) and use all of the onscreen functionality. Going into the source editor and manually editing caused some difficulty. Once I used the insert buttons on the right hand side, the system worked like a charm.

Just posting this because I went the long way...figured someone else might stumble onto this as well.

Thanks!