After you created the sketchflow project all you need is 4 simple steps:
1) Open the solution in Visual Studio
2) To the Screens Project, Add a package library reference by nuget to the mvvmlight package
3) Back in Blend, build, select the user control of your Screen_1.xaml
4) In Common Properties / DataContext / New / DataBinding / set the DataSource to the Main Property of the Locator
The latest step has to be repeated for each new view you need.
More clear informations here: http://blog.galasoft.ch/lbugnion/archive/2011/05/07/mvvm-light-v3-on-nuget-now-with-wp7-support.aspx