If you are new to Code Collaborator your first interaction with it might be to participate in a code review that someone else has created. In that case, you will get an email message (assuming default preference settings) when the author of the review is ready for feedback. The email message will contain a link to the Review Summary Screen.
Clicking that link opens the Review Summary Screen in your browser. The first time you see the screen it can be a bit overwhelming. There are multiple sections and on first experience it is only natural to ask "what am I supposed to click in order to get going?"
The answer is simple; scroll down to the Review Materials section:
This is where most of the "good stuff" is located. :-) The files available for review are listed in this section. Clicking a file name (in the example shown, Prime.java or PrimeUtils.java) will open the Code Collaborator Diff Viewer.
Once you are in the Diff Viewer you can do the real work of reviewing the changes, making comments on specific lines of code, opening defects on a specific line of code, etc.