This article assumes your have already created your releases in the "Release Management" section. If you have not done this please refer to one of the below articles.
To add a release to a "Contract In" please follow these steps -
1. Select "Royalty -> Contracts In" from the top menu
2. Select your contract from the list
Note - You can only add a "Release" to an "Artist Contract". Remix contracts are for "Tracks" only.
3. Click "Add Release" from the "Releases" tab at the bottom of the screen
4. Search for your release and select it from the list
5. Click the "Save" button
Your release will now be added to the release list.