When you make a query...do not chose a table...then go to the query section of the menu bar and choose UNION QUERY. It will pull up the SQL Page...
Another way is: in the design view in the left hand corner, you should see the data sheet symbol with a drop down arrow beside it. if you click the drop down arrow, you will see where you can choose SQL.
My suggestion is choose the first table you want with all the fields. then click the SQL button. That starts the code off for you. Access goes a bit crazy by specifying the [tablename].[fieldname] for everything. You don't need to do that, but its a start at least