Here's a screen shot:

Create a report parameter to hold the value then refer to the value of the parameter in a textbox in the report body. I was not able to test...

Is this parameter linked to a dataset?

If not you can create a boolean parameter by changing the data type to Boolean. Under Available values, choose Non-Queried and...

I was not able to find a member in the globals collection that references the data source. This is probably because you can have multiple datasets in the report...

Try the LookupCube() Function. Here is the syntax:

LookupCube(cube_string, string_expression)

example:

LookupCube("Budget", "[Account].[Total Expense]")