It looks like PostSharp relies on some MSBuild magic to invoke an executable to instrument the target assembly after compilation. LINQPad doesn't use MSBuild - it just compiles C# code with references, which means the target assembly won't get specially modified.
Compiled queries is for LINQ to SQL. Are you using Entity Framework? If so, this will not work. With EF5 and later, queries are automatically compiled in subsequent executions, so you won't need this anyway.