I am developing a Windows service that exposes a WCF service intended to be consumed by a Silverlight application, as explained in this blog post: http://mtaulty.com/Communi
You can find the configuration with the below steps.
Right Click Project in Solution Explorer
Choose Properties
Choose Compile TAB on the left
At the end of the compile page click Advanced Compile Option*s Button, this will bring up the A*dvanced Compiler Settings
The last control is a drop down reading "Target framework (all configurations)