By simply saving them into and opening them from your DropBox folder.
When using Dropbox, it's recommended that you go to Numbers preferences menu and deselect the option "Save new documents as packages", as it will be more convenient then, to access those files directly from the DropBox website. Else they will be saved as a folder by DropBox, and not as a single file.
That's for the Mac.
For iWork on iOS, I found this:
In my experience, you can Open an iWork document that resides in Dropbox with your iOS iWork app, but you can't use your iOS iWork app to store a document into Dropbox, or to update a document in Dropbox. The easiest way I can find to do this is to email the document to myself and then use my PC to update the Dropbox file. If you want to maintain a higher security, use your PC to move the document from iCloud.com to Dropbox.
Actually, its very easy to upload any given iOS iWork file right from within iWork back to Dropbox. Just select the printing&sharing option in any given iWork app, then select the option to open the file with another app, choose your export format (e.g Pages, PDF, Word), the select Dropbox as the app you want to open the file with... next thing you'll see, is Dropbox app starting and asking you under which name and in which Dropbox folder you want to save the file to. That's it.