You can't do this kind of look up with repeating fields. You can get the results you need if you use a table of related records with non-repeating fields.
Can you help out a little more Im slightly confused. I thought that if I related the tables and made the FirmID stored as Indexed and the Business_Firm_Name as a lookup this would work. Please give me some more detailed advise if you can I'm very new to filemaker.
If you have two repeating fields with 5 repetitions
Item[ ][ ][ ][ ][ ]
Name[ ][ ][ ][ ][ ]
Filemaker won't let you set up a lookup so that entering a number in a repetition of Item looks up a matching value in the same repetition of Name:
Item[ 1][ 4][6 ][ ][ ]
Name[ Apple ][ Orange ][ Peach ][ ][ ]
Instead, define a table of related records. Then you can set up your look ups without using repeating fields (Repeating fields are nearly always the second best choice for setting things up in your database. They DO have their uses, but only go certain specific situations.)
Have you ever set up a portal? If not, look up that term in filemaker help and see if you can figure out how to list your records in a portal. Then, if you're still having trouble, post back here for more explanation.