To add and item to a Touch Menu go to Pricebook > POS Touch Menu:

Touch Menu Listing displays all existing Touch Menus. The SITE USE keys are available for your use. Click on one of the site use keys to open:

To add items to a SITE USE key simply begin typing the item number or description in the field next to the desired key position. At least three characters are required before the system will display matching results. Click on the matching item to add.

Once the Touch Menu has been updated, it will need to be sent to the pos. 

Open your POS Touch Menu screen in S2K:

Click the PUBLISH button:

A window will pop-up, make sure your store is selected, and you are updating the Menu ID for ALL ITEMS in the Change Scope. Then click OK to save and PUBLISH the Menu to the register.

Log out and log back into the POS for the updates to take effect.