If you mean admin permissions in the Admin Control Panel, then that's in Users > Administrative Permissions on the bottom of that drop down.
If you arent a Super Admin, then you'll have to set your user ID in the includes/config.php file.
I am super admin, and I know that, however in order to be able to see that menu, the user you're editing must have an admin group set as his MAIN group... And again, in that menu you can't set admin privileges for ONE forum, you set admin privileges for the whole board...