Hmm...yeah, I guess phpMyAdmin is escaping whatever is in that field, including backslashes, so there's no way to put in special characters (or if there is a way, I can't figure it out).
But maybe try this instead: use the SQL tab and sql like
Code:
SELECT * FROM post WHERE pagetext LIKE '%\n\n\%\%%'
or
Code:
SELECT * FROM post WHERE pagetext LIKE '%\r\n\r\n\%\%%'