The distribution includes the file tools\nant\includes\ngenPostSharp.include. Include it in your build file as follows:
<property name="postSharp.dir" value="<Relative Path to Tools Folder>\PostSharp"/>
<include buildfile="<Relative Path to Tools Folder>\nant\includes\ngenPostSharp.include"/>
Then modify your compile target to depend on the ngenPostSharp target as follows:
<target name="build" description="Compile the application." depends="ngenPostSharp">
Now any time you run the build from Nant, ngen will check to see if it needs to update the pre-compiled image for PostSharp.