CPM (cost per mille) commissions are a special type of commissions that are rewarded for 1000 impressions (displaying your banners 1000 times).
Activating the feature
First you need to activate the feature from your merchant panel under Configuration -> Features. Find the CPM commissions feature in the list and click the Activate button next to it to activate the feature. Once the feature is activated you can set a new CPM commission type under your campaigns.
Setting up CPM commissions
Once the feature is activated, you can go to the Campaign manager and edit any campaign. Go to the Commissions settings tab and you'll see the CPM commission type.
The commissions for CPM can be defined in the same way as commissions for clicks and sales simply by clicking the Edit button of the 'CPM' commission type. The commission has to be set up as a fixed value in your system default currency so for example as $1. Once you've set the commission up do not forget to click the Enable button to enable the commissions.
Note: The commission can't be defined as percentage since there is no value to calculate the percentage from.