Try this:
Private Sub cmdUpdate_Click()
Dim db As DAO.Database
Dim rst As DAO.Recordset
Set db = CurrentDb
Set rst = db.OpenRecordset("Select Allocation from StockList where StockNumber =" & Me.StockNumber )
With rst
.Edit
!Allocation = !Allocation+ Nz(Me.ReqQty, 0)
.Update
End...