To fix the error I needed to change the where clause from this:
where PO.PO_Num = POItems.PO_Num and (PO.PO_Num = @PO_Num)
And add a question mark (?) in stead of the at symbol (@) like so:
where PO.PO_Num = POItems.PO_Num and (PO.PO_Num = ?)
After that I was able to execute the query, and Visual Studio 2010 didn't throw an error.