The easiest way to assign reviewers seems to be as a two-step process. First, give yourself some suggested reviewers using the “Manage Reviewer Candidate Suggestions” link. The reason for this is that you get to see the reviewer keywords easily. Note that if the other AC on this paper has already set up suggested reviewers, the list of suggested reviewers will already be populated with his or her reviewers. You need to add your own.
Once you've given yourself a set of candidate reviewers for each paper, go back to the main screen (probably easiest to click on “Select your role: Meta-Reviewer” and click “Manage Assignments, Bids and Conflicts”.
Alongside each paper it will say on the far right, “Edit Bids / Edit Conflicts / Edit Assignments”.
Clicking on “Edit Assignments” will bring up a list of your suggested reviewers. You finish the assignment by clicking the box along the right-hand side and then clicking “Save”. REMEMBER TO CLICK SAVE otherwise you will lose your assignments.
The reviewers will need to be notified that they have received their assignments and need to be let loose. CMT does not do this automatically. The best way is to mail them directly. (CMT provides a way for you to email your reviewers and fill in their papers, etc. But honestly, here be dragons.)