Don't know if you solved the problem but I had a similar problem earlier and solved it with help from this forum..
I added an dummy user from the mysql commandline..
FIrst logged in as root, start mysql console by typing mysql and press enter.
Say your dummy user's name is mysqluser, enter the following:
GRANT ALL PRIVILEGES ON databasename.* TO mysqluser@localhost IDENTIFIED BY 'password';
Databasename is the name of the database.. For full access, enter mysql.*
Password is the password you will enter in te config.inc.php
NOTE! All sql commands MUST end with an ; I missed that and had to spend almost a day searching the net for solutions :[
That worked for me, now I'm running phpmyadmin.