Accepting an iCal invitation

I received a meeting invitation from someone but I cannot accept the meeting invitation as I can’t use the Accept, Decline and Tentative buttons.
I believe the sender used iCal on a Mac to send me this meeting request.
Is there any way I can still accept this so it will go into my Outlook Calendar?

Support for this was first added in a post Service Pack 1 Hotfix for Outlook 2007 and later in Service Pack 2 as well so I suggest that you first check that Outlook is fully up-to-date before attempting this fix.

Once updated, you’ll have to make the following registry changes;

Key: HKEY_CURRENT_USER\Software\Microsoft\Office\12.0\Outlook\Options\Calendar
Value name: ExtractOrganizedMeetings
Value type: REG_DWORD
Value: 1

When this key is missing or set to 0, then you cannot accept meetings that have been sent to you in a 3rd party/unknown calendar format.
When this key is set to 1, then an attempt is made to interpret the 3rd party/unknown calendar format so you can accept it.