Based on the information in your last comment, I think the lack of donate button is indicative of a larger issue - this PCP isn't linked to a contribution page. I don't know if this is a data issue or a bug; I don't have any PCP-using sites running 4.6 yet.
If you know how to use SQL, try running this statement on your Civi database:
SELECT id, page_id, page_type FROM civicrm_pcp;
id corresponds to the PCP id you see in the URL for that page.
page_id should be the contribution page it's linked to, and
page_type should be "contribute". If they're not, you could fix the SQL directly, or figure out what caused the PCP to not be linked to the
contribution page. If this doesn't answer your question, you may want to post the results of that SQL as an edit to your question.