Sorry for the quick post of this morning, I was quite busy at the office and I couldn’t explain as well as I would like to.
First of all, you have to define your global var as always:
Name DataType Subtype Length
boolTest Boolean
Then, you add your global var to the DataItem that is used in your Tablix.
http://oi43.tinypic.com/6o33i8.jpg
If you have added the variable in the right Dataitem, you now can use it in the Layout.