Well there many be three possible reasons:
1. You downloaded MyBB 1.8. Mysidia's forum integration works only with MyBB 1.6, or alternatively you can check this post by Kyttias for how to get MyBB 1.8 to work with Mysidia:
http://mysidiaadoptables.com/forum/p...php?issueid=52
2. Your adoptables site is not fresh anymore, and already has more than 1 user. In this case, forum integration will fail no matter what. Note its stated clearly that forum integration only works if you run a fresh installation of Mysidia Adoptables, with only the one admin account existing.
3. You have configured forum integration incorrectly, the config vars are entered incorrectly. Check your forum database information and verify that they are correct.