Determining what to lookup

I am setting up a client database and have the address fields set up as follows. Here is one example of issues I am having:

ADDRESS TABLE

pk_AddressKey

City

State

Country

Region

CLIENTTABLE

fk_InvoiceCity

InvAddress1

InvZip

Lkp_InvCity

Lkp_InvState

Lkp_InvCountry

Lkp_InvRegion

fk_ShipCity

ShipAddress1

ShipZip

Lkp_ShpCity

Lkp_ShpState

Lkp_ShpCountry

Lkp_ShpRegion

The reason I have it set up this way is that there are several people using the database, and want to ensure that we won't have several ways of inputting the same city.

So the idea is that for both the invoicing and shipping information, we input the primary key of the Address Table and then the other information (city name, city state, etc.) are looked up.

I am having trouble with the lookup function however.

Lkp_InvCountry = Lookup ( : I have tried several syntaxes, but to no avail.

Thanks for your help, as I'm going to need this sort of operation throughout the database.