Is the phone brand new - have you set up a google account? You need one to access the market from this app. Settings, Accounts, add account.
If you do have an account then give this a try;
Go to settings, applications, manage applications, All (ie system + downloaded) find the market application then tap, tap "clear cache" then try that....
If that doesn't work go back into the same screen and tap "clear data" and try again.
This will force the app to reload everything (open it on wifi!)