There are two ways:
- Create a separate Google Account that happens to use the same email address as your Google Apps account.
- Upgrade your domain so that your Google Apps account is also a Google Account.
I would not recommend option 1, since then it becomes hard to maintain two separate accounts which look the same. Additionally, it will be much harder to transition to #2 once it is forced on your domain. Within a month or two, the 2nd option will be forced on all domains. I'd recommend speeding up this process if you need it right away.
Note that even with method #2, you cannot use all of Google's services yet. Here's a list of some of the products that you cannot use:
- Android Developer
- Extra Storage
- Web History