Hello Stephanie,
Activating V3.0 does not require a registration key or password. A license activation program is available for you (you must be logged into this site to see it) at the bottom of the Product Info >> Upgrading from 2.x or 3.0 page.
Your old username/password with Thayer will not work on this site or the Online Store. So simply register anew at both (as you have already done for this site).
If you have a backup/copy of your v3.0 database. That is all you need to install and run v3.0 on your Windows 7 computer or to upgrade that data to v3.6 if you decide to upgrade.
Instructions for moving BD to a new computer can be found here: http://www.birdersdiary.com/Documentation/BDV37/How_To/Move_Birder_s_Diary_to_a_new_Computer.htm
And of course, the "Upgrading from 2.x or 3.0" page referenced above has all the details you need for upgrading from 3.0 to 3.6.
Let me know if I can assist with anything further.
Jeff |