To clarify, you are referring to the original Developer preview? You should be using the Release Preview build which has that PowerShell cmdlet. The original Developer Preview used another process to obtain the developer license. The MSDN content on
that process has been removed with the new preferred PowerShell process.
If you must use DP, then you might look for some old forum threads such as
this.
The VS release for DP would automatically aquire the developer license for you as well.
Thanks!
David Lamb