How to select multiple machines into a new item type

Hi folks, i'm working in ARAS for my university project. I need some help from you guys. Project is to develop a platform in aras which is responsible to show sequence of operations(activities) with corresponding machines for every product type in a company. so i started to do in process plan and  i created an item type named activities where list of activities are displayed. My question is how i can select multiple machines when i'm creating new item in activities and how can i link this item type named activities in process plan under 'add operation'. when i want to add new operation,  i want to choose directly from activities. i'm doing in Aras Innovator Version 11.0 SP12