HipChat must be connected through the Manage page.
For paid accounts, this page is only accessible by the account owner. For free accounts, this page should be accessible by any team member.
When you visit the Manage page, you'll see something like this:
Select HipChat. You'll see this:
Type in the Auth Token and the room name where you'd like WorkingOn posts to go.
Hit "enter," and setup will be complete! You should see a confirmation message in HipChat.