You must have a minimum of 50 credits in your account to start a Campaign. Once you have enough credits, do the following:

Click on 'My Campaigns' in the sidebar:

Then click 'Start New Campaign' at the top of the page:

You will then be asked to select which track/playlist to use, from your public tracks on Soundcloud:

The next step is to set your campaign's budget - this determines maximum reach of your campaign. For instance, a 50 credit campaign will have a maximum reach of 5,000. This is because the cost of a repost is 1 credit for every 100 followers a reposter has.

In the same window, you can also enable CommentPlus, and if you are a Pro Plan subscriber, you will also be able to use the Campaign Accelerator and set Custom Campaign Rules: