I have an instance running on Amazon EC2. I need to point my domain
example.com at 1und1 to this instance. How do I do this?
I'm not familiar with 1&1's interface, but it looks like you can set up A records using their DNS service. You should follow the instructions on their page to point your domain name to an IP address.
You can find out what your instance IP is in the Amazon EC2 console. Click on an instance name in the instances list. Directly below the name will be the public IP for that instance. I have attached a screenshot for clarity:
In this example, the public IP address is 249.212.101.96. This is the IP address you should enter in Step 5 of the instructions on the 1&1 site.