Question for Ken Sheridan re conditional warning message

Ken -
On March 2 you kindly gave me some advice re the code to use to produce a
warning message box if certain conditions were met on opening a record.
It worked beautifully on a simple little test mdb but now I'm trying to put
it into our corporate database and it errors. I think it's something to do
with the fact that the values that it checks against are coming from another
table and I may not have got the syntax correct. I took note of your advice
re combos and referenced tables but couldn't resolve it. It may be because my
structure is a bit messy, as bits have been added on over time. Here's the
situation in detail:
The form being opened (when the message is needed on Current) is Main and
it's underlying table is Inquiries.
1. On this form there is a combo named InquiryType. This looks up the table
InquiryTypes and allows selection of an InquiryTypeCode. There are 4 of these
in all and we need the message to fire (in combination with point 2 below) if
the InquiryTypeCode is either "I" or "S". (these represent persons not having
any credit facilities).

2. The form includes a tab page named Dates which carries an Events subform
based on the Events table. This subform includes a control named PaymentDate,
which is a date format and which users complete when we have received
payment.

So.... the message would need to fire when (a) the InquiryTypeCode is either
"I" or "S" AND (b) the PaymentDate field is null.

Sorry about the lengthy explanation but I hope it may enable you to give me
the exact particular code that I need to use under the circumstances.
This could be worth a beer or two next time I'm in Stafford...
Many thanks!
CW