I don't think you can, for all profiles, but if you select 300dpi and make a scan then it remembers that, and the profile used, for the Custom Setting.
If you want to continue using TWAIN follow the instructions at http://helpx.adobe.com/photoshop-elements/kb/twain-installed-photoshop-elements-11.html.
Cheers,
--
Neale
Insanity is hereditary, you get it from your children
If this post or another user's post resolves the original issue, please mark the posts as correct accordingly. This helps other users with similar trouble get answers to their questions quicker. Thanks.