You didn't mention how you downloaded it. PSN store? Xbox marketplace? what?
If you are talking about xbox live marketplace then follow these instructions:
View your download history and download content again
You can view a list of what you've downloaded from Xbox LIVE on your console or your computer. You can choose to download an item again (if needed) for no additional charge. Here's how:
On your console
- On your console, sign in to Xbox LIVE using the gamertag that you used when you originally purchased the content.
- Go to My Xbox, and then select your avatar.
- Go to Account Management, and then select Manage Account.
- Select Download History.
- Browse your download history. If you want to download something again, select the item, and then select Download Again.
If you want to see the charges on your Xbox LIVE account, please see Check your Xbox LIVE bill and download history
On your computer
- To see your download history, go to live.xbox.com/Download/History and sign in with your Windows Live ID (email and password).
- Browse your download history. If you want to download something again, click Add to queue. This adds the item to your console's download queue.
- Turn on your console and connect to Xbox LIVE. The items will begin downloading.
Downloading previously purchased content to a different Xbox 360 console does not transfer the content's license. If you want to use content without signing in to Xbox LIVE, you must transfer the content license to the console. Learn how to transfer content licenses