If you searched for "No servers could be used! Exiting" and wound up here try a couple of these things. They helped me

1. Make sure you're not including the http:// part in the guiminer host
2. After the address in the host field, do not include :3334  the colon and the number behind is is nomenclature for the port, put that number in the port line and delete the colon.
3. Make sure the username is your pool username (dot) your worker username, see any youtube tutorial on how to join a pool.
4. Do not use the default user in your guiminer, create a new miner by going to file > new miner > cg
5. Don't use a complicated password for the worker. One letter ought to do it. The letter goes through in clear text anyway. If someone wants to mine for you, let them. I actually had a super secure password for the miner, and it just wouldn't process.  Do however make a secure (different) password for what ever mining pool you join.

Hope that helps, and if it does, send me a lite coin thank you. LU97CfKNgRCVTQmShRwzKFA63fD7XLuJsc