Export Dataset can only take one query at a time.
To get around that, make multiple Export Dataset actions - one for each query. If you want them to all have the same properties (except the query), a fast way to do that is copy/paste the action in the automation desgner.
You can then run them all by right-click on the App (on the left side of the Automation Designer) and choosing "run".
I don't think this was in Toad 11, but in Toad 13, there is an option to add a new sheet in the Excel file instead of overwrite.