[DJMaze]

DataSource: a dataset (table)
DataField: a field of the dataset

Lookup: another dataset
LookupField: a field of the other dataset
LookupKey: a field of the other dataset

As you can see the word "lookup" was the hint in my first post and is important.
Therefore there are two kinds of DB combo boxes.

1. a dropdown based on strings
2. a dropdown based on a Lookup dataset

#2 is the advanced version where you see column X but it will store Y as value in the datasource.

If this is still all new to you and you are using the free version of MS then i doubt you have the advanced components.
Then you must use #1 and fill the dropdown programaticly and store the value programaticly.