This article assumes that you are currently running Pidgin as an IM client.
From the Pidgin menu bar choose Accounts and Manage Accounts (CTRL-a). From the Accounts window choose Add.
Next it will prompt you for your username, your Domain and your Password. Your username is the username portion of your SIP address. For example, if your SIP address is firstname.lastname@example.org, "joe" is your Username. "example.onsip.com" is your Domain.
Pidgin will prompt you to accept the Junction Networks / OnSIP certificate. Choose Accept when prompted.
Note: If you run Pidgin AND my.onsip.com, replies will appear in both windows.