Database doesn't exist yet, I'm in planning stages and just playing with the
basic table structures, and right now I'm stuck on this seemingly simple
problem:

Users would like to be able to select from a list of filter types (oil air,
fuel, whatever) on a form but want to be able to add to the list. With no
referential integrity, it allows users to type anything they want in the
textbox but doesn't add to the lookup table. Set updates, and I can't seem
to get the relationship right so that it updates the lookup table - it
errors with no matching record. Do I need to create a button which calls a
form for them to add the record first, and then select from the lookup list?
I know that will work, but it seems there ought to be a simpler way (for the
user) to accomplish this.

What am I doing wrong?

Thanks in advance, I'm sure I'm missing something obvious here.

SusanV