Thank Comment,
Your solution doesn't work for me. I've tried to simplify a little more the situation, I think..
Table A has those fields: pKTableA (primary key) and valueA (text value).
Table B has those fields: pKTableB (primary key), fKTableA (foreign key populated with pKTableA), other values..
When I was populating table B, I use checkbox input format (that allow to select multiple values) for the fKTableA, showing the related value (valueA) in the field itself. Now, when I try to create a report, filemaker correctly show me the content of fKTableA: the numeric key of table A. I would like to show, instead of a list of numeric values for each record, the list of related text values.
The calculation field should make a calculation like this: For each line of the field fKTableA get the number and substitutes it with the related text value.
I hope that I explain it more clearly.. Thanks in advance