Right the project then click to open .
To add activities click the + icon at the upper-right screen so that you can add new new activity. Then just click Next Next Next until Finish but of course you have to read them so that you can learn what is being asked about in each dialog, or you can just click Finish right away. Repeat this process until your finished adding the new activities.
See the video for additional knowledge on adding activities in Primavera P6.