I guess it is because the EXTRA FIELD IN CUSTOMER ACCOUNT mod appears not to have been updated for 1.5.1. That mod replaces the create account module with it's own version.
I can see two ways to go.
1) Because the extra field mod is using a 1.5 version of the create account module (/includes/modules/YOUR_TEMPLATE/create_account.php), you could try using the 1.5 instructions for my reCAPTCHA mod. or
2) Use the 1.5.1 version of the file and incorporate the changes necessary for the extra fields mod into it..

I think option 1 is probably easier.
Regards