Select the checkbox of the tasks to which you want to assign resources.
Click Assign to....
If you are assigning an internal resource:
In the Available Resources column, select the checkboxes of the resources that you want to assign to the selected tasks. If you want to assign all available resources, select the checkbox next to Name.
Click +.
If you are assigning an external resource:
In the Add External field, enter the name of the external resource to which you want to assign the selected tasks.