Liz
I would need further details. Where are you in your project when you get the message about the automatic message about "...add file to end of my movie"?
In your Premiere Elements 12 Mac, are you working in the Expert, Quick, Guided Expert, or Guided Quick workspace? If one of the Guided choices, please let us know what lesson you are following?
Generalizing....
a. Import your video into the project with Add Media/Files and Folders/Project Assets.
b. Assuming that you had a video clip already on the Expert workspace Timeline, hold down the Mac Option key as you use the mouse cursor to drag the video from project assets the right edge of the clip that is already there and down. Release the mouse click.
(I am strictly an Elements Windows user, so I am translating the Windows Alt to Mac Option. Does Mac Option work?)
We will be watching for further details and results.
Thank you.
ATR
