The longish thread on DropN900 actually contains all the instructions you need, but it is longish, and they are spread out. And it took me a while to get used to the interface of DropN900, it is really unintuitive.

I do this with a Mac, which makes it really easy, since it has a Unix-like OS and thus supports symlinks in much the way the N900 does. You create your Dropbox folder in the N900 above MyDocs, e.g., at /home/user/Dropbox (since MyDocs does not support symlinks) and when inside that folder, from x-term use ln -s with the path to the folder you want to sync. Then do the same on the Mac side. Then in DropN900, you just have to press 'Sync' and follow the prompts and it will upload/download to keep the folders in sync. On the Mac side you will see little pop-ups as the files come in!

There are caveats and restrictions. Firstly, I have not tried to do this with large folders. If you want to do it with large folders, I would copy them first and just use Dropbox to keep them updated. Secondly, when DropN900 sees a file on the Dropbox server that it does not have, it does not know whether the file was created on the N900 or deleted on the server side. This is not a limitation of DropN900 as far as I know, but of the API that Dropbox(R) makes available to the world. Sometimes it will prompt to delete the local files or download the remote ones, sometimes it doesn't and just wants to download the remote ones, so this requires some care. Thirdly, in Windows symlinks are not as easy as on a Mac, but still quite possible; see here for example. Fourth, DropN900 handles media files stored on the device differently; you sync these by <Main Menu> -> Sync Media. It only syncs media on internal storage (not on the memory card) and a user has extended the functionality to allow local deletion of files not on the server (see the thread), but there is nothing to stop you from also symlinking your media files to the Dropbox folder just like any other files.