You can create new Advertisers by navigating to any of the Agencies in your Megaphone account.
1) Click on "Add Advertiser"
Clicking this button will open a modal where you can enter the Advertiser's information.
2) Enter Advertiser Name
3) Set Competitive Categories
All of the IAB's Categories and Sub-Categories are included in the modal. You can select as many as make sense for each Advertiser. The categories are used for competitive separation - Advertisers with the same category associated won't run in the same podcast/episode. This is especially important if you are participating in the Megaphone Targeted Marketplace.