Open an emailed DropBox-link in Finder

If I understand what you're asking, likely the file:///path-to-file syntax is what you'll use.

The short answer... is NO it cannot be done.

For a number of reasons Dropbox do not support this.

The path to a file on your computer will not be the same as the path to a file on someone else's computer, even if that file is in the same shared folder. By default, each user's Dropbox folder is located within their computer user profile, which often include that person's name or username in the path. That means a link to a file on your computer will not work on anyone else's computer.
Also people can freely rename shared folders to anything they want without impacting the functionality of the share, and would completely break a FILE:// link.

You could create a RELATIVE path link on OSX if you were 100% sure that the Dropbox folder was in the same location on Everyone's computer .. ANY small difference in naming or the location of the Dropbox folder and the link will simply not work .. hence the reason Dropbox prefer the URL method

file://~/Documents/Dropbox/FolderName/Filename.doc

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You're supposed to share the dropbox link or folder from dropbox to another dropbox user via emali.  The email link opens up a browser to accept the share.  Once it's been accepted, you can see it as part of your dropbox in finder.

Just for clarification, are you referring to the shared Drop Box folder in your Users folder on all Macintosh computers, or are you referring to the commercial sharing service Dropbox™?  I believe David and Eoin are answering in reference to the first, while serialband is speaking of the second.