The diffrent settings in windows messenger is controlled in the regestry. You can simply save the following file to a text file named enablesip.reg
---------------------Do not include this line -------------------
Windows Registry Editor Version 5.00
[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\MessengerService\ServiceProviders\{83D4679F-B6D7-11D2-BF36-00C04FB90A03}]
"Enabled"=dword:00000001
"Primary"=dword:00000001---------------------Do not include this line -------------------
as an alternative you can download a premade file here
Now restart messenger and it would show up as intended.