Trying to pull data from other worksheet using a formula dependent on a drop down list.

I am really frustrated - I cannot seem to get the formula/settings right to show the info I want to in my sales report.

Basically I am showing number of units sold by unit design by month for 2012. What i would like to do is, by using a drop down list, filter that information by Regional Sales manager. But I am totally stuck. I have the initial formula right, but when it come tro trying and involving a drop down list I am very confused - I have attached a sample of what I am looking at. Any help would be greatfully received (the drop down is in A5)- (Dont worry about the #REFS in the other months... i just removed the rest of the figures for this example)

Thanks guysExcel help.xlsx

I also want to have an "ALL" or blank selection where is shows all the sales data

Thanks