Running OSX Yosemite on a MacBook Pro
Downloaded latest version for a Trial
Can you say exactly what error message you see?
Make sure you have the latest installer:
If it still fails, after it fails can you start the console (type Console into spotlight at the top right)? Can you copy and paste the last couple of pages into an email to us?
Also are you able to try signing in as a different user and installing it under that user? Does this make any difference?