Active members of XtGem's forum are already aware of this functionality, but I have decided to repost it here for convenience.
First off, register an account on the chosen advertiser's site, then include one of these codes, changing siteid (and zone, in case of mojiva, publisherid and adspace_id in case of smaato) with your provided account id. You can find these parameters either somewhere on their site while logged in or extract them from one of their provided integration codes (PHP/ASP/etc).
You can include as many ad service codes as you want on your site, max one per network and free ads will be returned after the first non-free ad is returned from the service.
So for example if you have these codes:
If admob doesn't return an ad, nothing will be displayed in the first code instance and buzzcity code will be executed. If admob does return an ad, buzzcity will not be executed. Second admob instance will not be executed either way, as it's a duplicate call.
It is also very important to note that you might not see ads on your site at all. This is not XtGem's bug! Either your site has not been accepted by the ad network (you can usually check your site's status on ad network's site) or they do not have any ad inventory with targeting matching your country/browser. Despite that, your site visitors might still be seeing ads. It's best to check ad network's site for stats and see if ads are being delivered.
has compiled a nice summary of various ad networks on his site here
If in doubt, try asking for help in the forums