If the aspect ratio of an imported video file is not recognized correctly, you can adjust it manually. Just right-click on the imported object's video track in timeline mode and bring up the "Object properties" dialog. Manually set the aspect ratio to 16:9 in the appropriate drop-down list and confirm the dialog by pressing OK. Make sure to set the preview monitor to the matching aspect ratio, so the video gets displayed correcty.
Ralftaro is right, i import all my video from another computer and its 16:9 but seen as 4:3. All I had to do was in the time line, right click the vide. Then click on 'Object Properties' then change the 'Aspect ratio' to 16:9. then it was the way it should be!