Add the variables to the root page of the Slim tests. This could be the root of the FitNesse wiki (e.g., localhost:8080/root) if we're only doing Slim tests, or on the Slim test page or suite page if we have a mix of Slim and Fit tests in the wiki.
You may have Java paths defined. Make sure the FitNesse root (e.g., localhost:8080/root) and any other pages don't contain Java paths. If you find the following, remove them.
!path classes
!path fitnesse.jar
!path fitlibrary.jar
From http://fitsharp.github.io/Slim/GettingStarted.html
If these variables are in the set up page, FitNesse is already processing the page as a Fit Java page before it reads them, instead of a Slim .NET page.