That all sounds ok, but your pasted sample has several issues.
Line 1. should be <Project ToolsVersion="3.5" DefaultTargets="Default" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
Line 2. should be <Import Project="$(MSBuildExtensionsPath)\ExtensionPack\MSBuild.ExtensionPack.tasks"/>, note the 's' in MSBuildExtensionsPath
Line 3. should be </Project>
Prob just some copy paste errors as your error indicates whatever you have is compiling. I can only think that Line 2 is your problem, but your error message has me a bit confused as it indicates a correct path.
Have you made any progress?