There is really no way I can debug this for you.
You are goign to have to check everything.
1) you do know the file is plugins.ini NOT plugins.txt right?
2) You have paths in your plugins file so you know you have to have those paths setup correctly where your dll files are.
3) All I can say is go here and read all of this on how to install and test Metamod.
http://wiki.bots-united.com/index.php/Main_Page
Scrool down to the Metamod section.
Good luck!