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Do you have the ability to execute SQL queries on your MySQL db directly? If so, try the following:
Query #1 (get uid of your user account)
SELECT uid FROM `<table prefix>_users` WHERE uname='<login>'
Write down the uid returned by the above query, normally it will be '1'.
Query #2 (add user to admin group)
INSERT INTO `<table prefix>_groups_users_link` (uid, groupid) VALUES(<value from query #1>, 1)
For the above queries to work you'll need to replace the values enclosed in <> with your installation specific values.