If you are unable to find an item against the supplier this may be because the items were created via stock check.
In order to create a supplier return, go to the 'Movements' module and select 'Supplier Returns'.
Enter 'Avail. Lines' and search '%' in the company search box and select 'Search'.
This should bring the items up with no supplier set up against them.
In the company search box type in the correct company name and select the 'Search' in the top left corner so that both the account code and name fields are populated.
Highlight the required lines and select 'Create Notes'