Sign into the same ID you are using for iTunes Match on your Mac by opening iTunes, going to Store>Sign Out, the Store>Sign in with the other ID. Then turn on iTunes Match by going to Store>Turn on iTunes Match, then Add this Computer.
Note: If your Mac has automatic downloads enabled now in iTunes>Preferences>Store, or if you have download previous purchases from your iTunes purchased list, it is already associated with the current ID. Once associated with an Apple ID in this way, there is a 90-day wait period before you can associate it with a different ID. What this means is that you may not be able to enable iTunes Match on your Mac with the other ID for a period of 90 days. This is discussed in this article: http://support.apple.com/kb/HT4627.