At the moment you can only run one installation of Dropbox on a single computer and it can only be linked to a single account.
Accessing another account from a computer where Dropbox is already installed (or any computer, for that matter) is as easy as logging in as another user through the Dropbox website.
Otherwise, you can link accounts together by creating shared folders. Shared folders are designed to allow you to sync files between multiple accounts.
Another option is to use a different Dropbox account for each user login on your computer. While this technically means you can have several Dropbox folders on the same computer, you will have to switch between each user account to take advantage of Dropbox's syncing features. This method is best for groups or families that have individual Dropbox accounts and use unique user logins on the same computer.