Give this a go:

B = DLookup("Balance", "SeedStockT", "ProductID=" & ProductCombo & " And SeedDate=#" & LastD & "#")

Thanks Kevin, that worked. I got close, trying combinations of ",# and & but never quite hit the mark.
I presume I now need to run SQL to actually do the update.  Can I use the DIM variable B in the SQL statement?

I know how I can update the balance through a couple of (invisible) textboxes in my form but that is not very elegant.
I now have the correct table balance in my DIM variable B.  I have the seeds used in my form.  I now need to deduct the seeds used from the table balance and insert the new balance with the correct date and ProductID in the table. I presume SQL is the way to go. I got this but it didn't work, I suspect I can't use a DIM variable in a SQL statement:

Dim B
    Dim NB
    Dim LastD
        LastD = DMax("SeedDate", "SeedStockT", "ProductID=" & ProductCombo.Column(0))
        B = DLookup("Balance", "SeedStockT", "ProductID=" & ProductCombo.Column(0) & " And SeedDate=#" & LastD & "#")
        NB = B - Quantity
        DoCmd.RunSQL "INSERT INTO SeedStockT(SeedDate,ProductID,Balance)" & _
       "VALUES(#" & StageDate & "#,"ProductCombo.Column(0)","NB")";

Your can use variables. You're missing your ambersands (&) around the productcombo

Got it to work, forgot the & where they were needed

You're missing some ampersands (&).

DoCmd.RunSQL "INSERT INTO SeedStockT(SeedDate,ProductID,Balance) " & _
       "VALUES(#" & StageDate & "#," & ProductCombo.Column(0) & "," & NB & ")";

Adam beat me to it. LOL

Thanks guys. At least I worked it out myself before reading your posts.
Here is the working Sub.  Any comments for improvements?

Private Sub Quantity_AfterUpdate()
    Dim B
    Dim NB
    Dim LastD
        LastD = DMax("SeedDate", "SeedStockT", "ProductID=" & ProductCombo.Column(0))
        B = DLookup("Balance", "SeedStockT", "ProductID=" & ProductCombo.Column(0) & " And SeedDate=#" & LastD & "#")
        NB = B - Quantity
        DoCmd.RunSQL "INSERT INTO SeedStockT(SeedDate,ProductID,SeedsUsed,Balance)" & _
       "VALUES(#" & StageDate & "#," & ProductCombo.Column(0) & "," & Quantity & "," & Val(NB) & ")"
End Sub