The latest version of Sibelius (version 8) no longer functions as a free player, so I’d recommend sticking with the older version (version 7):
There are also good tablet versions available, including an iOS application “Avid Sibelius Scorch”. I would highly recommend this if you have an iPad, it’s only a couple of pounds and works very well.
There are some slightly baffling aspects to the user interface on the PC version, I’ve highlighted the most useful features below:
Controlling the Sibelius Client
In Sibelius, click on “View” on the top menu to select the playback controls – check ‘Transport’ and ‘Mixer’:
Now, when you open a score (.sib file) you will be able to control playback and tempo from the ‘transport’ control panel, and select parts from the ‘mixer’: