I've upgraded to iTunes 10.5.1, which the release notes claim includes the widely publicized iTunes Match feature, but I can't find any way to purchase and enable it. How do I enable iTunes Match?
If you're located outside the US, iTunes Match is unavailable for the time being, due to content licensing deals.
When iTunes Match was first announced I believe Apple stated they would be working on bringing it to more countries, but these negotiations always take some time. Hard to give a definite timeframe unless Apple announces something.
Users outside the USA as has been mentioned will not have the service due to licensing however for those in the USA searching for the answer, and (if) it rolls out worldwide, you can set up iTunes match by clicking on iTunes Match just under the iTunes store in the left had plane and follow the steps.
You will need one of the following devices running iOS 5.0.1 or higher,
iPad / 2 iPhone 3GS or above iPod Touch 3rd Gen or higher
You will get an option to use the paid for service as a yearly subscription or just use the free version.
iTunes match was released worldwide in December 2011