Sorry for the many questions....
Where did the source files come from?
Where were the source files when you started the project?
Where are the source files now?
What is the frame rate of the source files? Is it variable?
What exactly does the message say?
Can you post a screen shot of the message?
What version of Premier Elements are you using?
Do you have Apple's Quicktime installed?
What computer and CPU do you have?
What computer operating system are you using?
How much disk space do you have?
How much memory do you have?