To add handsets to a specific event, head to the Edit section of your chosen event and look in the Handsets tab on the right hand side.
This column will display all handsets available to you as assigned to your account (both online and offline). Tick the boxes next to the handsets listed that you wish to have assigned to your event.
After exiting edit mode and opening dispatch for the event, the menu on the right will now be populated with the handsets selected in the previous step. You are now ready to begin operations for this event.