If installed, the default SMTP virtual server can be found by clicking Start --> Run --> "inetmgr6" --> OK. Microsoft did not upgrade the SMTP service from IIS6. If you can't get to it, then you will need to install it as a role feature.
In IIS7, click the site you want to change and click "Basic Settings" in the top right-hand corner. In the window that pops up, you'll see the name of the application pool. Then go to the application pool window, and change the .NET version from there.
There is no Web Service Extensions option like in IIS6.