I'm assuming you have a form on the landing page. The way we do this is to add a hidden field to the form & then add the value to be entered for that hidden field in the URL string.
For example:
Form has usual fields and a hidden field named Lead Source Description with the URL Parameter of LeadSourceDesc
URL given for the campaign would look like http://yourmarketoinstance.com?LeadSourceDesc=AdCampaign1
This will only track the performance of conversions and not hits from the ads, obviously. To track the hits, you'd need to do something with your web analytics package as Charlie mentioned.