Trajectories are shown with the  MD Movie tool (start it by choosing Tools... MD/Ensemble Analysis... MD Movie from the Chimera menu).  That gives a file browser for opening trajectory formats.

MD Movie documentation includes a list of the possible formats, and much more about the tool:
<http://www.cgl.ucsf.edu/chimera/docs/ContributedSoftware/movie/framemovie.html>

Also, the following tutorial, part 1, uses MD Movie to view a sample Amber trajectory (data provided along with the tutorial):

<http://www.cgl.ucsf.edu/chimera/docs/UsersGuide/tutorials/ensembles2.html>

(Part 2 also uses MD Movie, but on different data, an NMR ensemble from the PDB.)

The documentation and tutorials are also included with your download and can be accessed from the Chimera Help menu.  I hope this helps,

Elaine