I suppose you mean
SiliconStudioXenkoDir environment variable. Normally if you open the solution from the GameStudio it will be set automatically to the current version used. You might want to set it if you want to use compiled sources instead of the installed assemblies.
Depending on the tool you use the syntax can be different.
In Windows command line console, you can set it using this syntax:
Then if you launch Visual Studio from the same command prompt it will use this overridden value instead of the v\one defined globally in the system.
In VisualStudio project files (.csproj), the syntax is a bit different:
<SiliconStudioXenkoDir Condition="'$(SiliconStudioXenkoDir)' == ''">C:\YourPath</SiliconStudioXenkoDir>
Condition="'$(SiliconStudioXenkoDir)' == ''" means the variable will only be set to the value
C:\YourPath if it was empty or unset until now.
To use it in a path (e.g. project reference), your resolve it with the
But you shouldn't have to worry about that if you still use the .csproj generated with the GameStudio. Settings the environment variable (like explained above) should be sufficient.