When you join a server running the itemserver plugin for the first time, you'll get prompted for a username and a userkey. These are not
the values you used to register. You can find the right data to log in under the 'profile' tab.
You can click the fields or press tab to change focus and press enter to confirm. You can paste text in the fields by right clicking or pressing ctrl+v.
This is how it should look (with your own info of course):
If you accidently closed the login window you can re-open it by pressing F5.