If you want to apply a lookup to a field then the triggering key field has to be an indexable field in the same table.
In your example there is no stored key field in table B which can trigger lookups in that table.
Dear Jens: Jan. 12th, 2009
Thanks for your reply. I guess you can't define a key where it is
stored as a related value from another table. However, I did discover
that if I use the Table_1 to Table_of_Info relationship for defining fields
in Table_2, I can actually get the values that I need. This may not be
exactly proper to do this. I think what happens is that Table_1 and
Table_2 are related through the Index values (through the Main_Table)
and Table_2 is related to the Table_of_Info through Field_A. Evidently
Filemaker tracks down the needed value using the two relationship paths.
I notice that if Field_A in Table_2 is changed, sometimes I need to refresh
Table_2 to update the Field_B value.