Under the recently released CodeSmith 5.1, main.cst fails to build, and the Properties window does not initialize with the custom properties.
 
Examination of the build output reveals;
c:\ProgramData\CodeSmith\v5.1\Template Cache\1876480247\Main_cst.cs(500,56): error CS0234: The type or namespace name 'CustomProperties' does not exist in the namespace 'CodeSmith' (are you missing an assembly reference?)
 
Solution;
add following to Main.cst
<%@ Assembly Name="CodeSmith.CustomProperties" %>