It seems that nightly build (2010-05-12) doesn't work with Contrib -- I copied the latest stable (0.85) NantContrib into the bin / tasks / lib folders as per the instructions. With this the <msbuild> task was not visible. I then copied the Nant.Contrib.Tasks.Dll directly into C:\Program Files\nant-0.91-nightly-2010-05-12\bin. Still; nant error message: Invalid element <msbuild>. Unknown task or datatype.