I'm trying to use the code review feature when I check-in my files but I get the following message at the top of team explorer :
To request a code review from your peers, enable the new features. Click here to enable the new features
When I click the link, it gets me to my project profile site, but I don't see any option to activate the code review feature anywhere...
Have a read of these two blog posts by Ewald Hofman, they cover the process of upgrading your team project or process template to allow new features.
In the event that the automatic upgrade cannot be performed, follow this MSDN article.