There is already a thread on this in the General Section.
Go to the tuxtime site.
sign up
login, and you will be given your computer id
download the client, put in your user/pass/ID
voilla.
Note: there is a bug in the current official client which causes it not to remember your password, simply fill in your details, send your uptime, and then click the [Quit] button.
Re-open the client and your details should have been written to the config file.