Cell reference based on another cell choice from a drop down list.

I have a workbook that I have created that I want to use to show what area an address is located.

I have 2 worksheets in the workbook
-- Locations (I plan on hiding this)
-- Site Lookup (which will be the main worksheet)

on the locations worksheet i have 2 columns,
column A - Sites -- which lists all the addresses
column b - Areas -- which lists the area that the address is located in

on the site look up worksheet
Column a -- I have created a drop down listed based on the "sites" column from the locations work sheet
Column b -- I want this to populate with the "area" that corresponds with the address that is chosen from the drop down list each time I choose a different address from column a.
I have this vlookup: =VLOOKUP(Sites, Locations!A1:B255,2, FALSE), but when I choose a different address from the drop down menu, it doesn't change.

I have attached my spreadsheet, hopefully it makes sense and someone can give some suggestions.

Thanks,

James