To select a variable into the workbench, select the Pointer tool and double click on the variable you want to select into the workbench.
As you are adding new variables to the sketch, the types of those variables might not be known until you actually enter equations. Thus the variable selection window will not always yield what you would expect when searching by type. Searching by name and wildcard pattern matches (e.g.. *labor*) is more likely to give you the desired results.
