If you haven't already created a query, go to the Queries tab of MS Access and click on the New option; I suggest selecting the Design View, then choose the table with the Description field and click Add, then Close.
In the first Field column, choose Description from the dropdown list.
In the second Field column, type--
Code:
Mid(Description,InStr(Description,"#")+1,6)
--which Access will change (after you've pressed the Enter key) to--
Code:
Expr1: Mid([Description],InStr([Description],"#")+1,6)
(You can go back and change Expr1 to something like CustomerID if you wish.)
Click the View icon to "run" the query, which will give you a list of the Description values and the CustomerID from each one.
Save the query.