You will find all flows to which you are designated on your Author-tab under Overview:
1. Choose the pencil for a flow where you want to add an activity:
In the window you can choose which activity you want to add.
2. Press the green arrow for an activity:
Notice how the chosen activity is shown at the top. If you choose more than one activity (Not possible for FLOWmulti) they will be added as secondary assignments. This means the manager of the flow has to distribute the assignments between participants.
3. Choose Save at the bottom.