You have two options here and you'll have to decide which option is best for your needs: Both options require you to set up a relationship that uses the field you've formatted with your drop down list to link records between the two tables.
Option 1: Physically copy the data from the other table. This is good when you don't want changes made to the other table to result in matching changes appearing in existing records in your main table. Looking up prices and sales tax rates from a related table are the typical examples of why you would set it up this way.
Option 2: Simply refer (via the relationship) to the data in the other table. In this option, any changes made to the data in the other table automatically appear for all existing records as well as any future records. Updating customer info in a contacts table is one example where you might want to do it that way.
Option 1 implementation:
Create a matching data field in your main table. In field options, select either "looked up value" or "calculated value". Either option may be used to physically copy the data into your main table's data field. The calculation option allows you to make additional modifications to the data besides simply copying it.
Simply use the field tool to place a copy of the field from the other table on your data entry layout for your main table records. When you select a value in your drop down that matches a record in the other table, the data will automatically appear.
Thanks so much for the quick reply Option 2 works the best for me and even better works a treat.... so simple!