First maintain the Promoter Commission Rate Maintenance...

Go to Point of Sale > Promoter > Promoter Commission Maintenance  

1. Click on New to add commission ID and rate. OK. (this is the general commission rate)

2. choose to filter by either item code, item group or item type, then click on Edit. Key in the commission rate for any selected row(s). OK.  (this is the specific commission rate for the selected group/type/item; if this rate is set to '0', it means to be excluded from general commission rate)

3. The selected item/group/type with commission rate will be displayed at the lower part of the screen.

Go to Stock > Stock Item Maintenance  

Check the checkbox of 'Has Promoter', so that when the item is being selected, a select promoter screen will be prompted.