Permissions issues with shared folder
The concept behind having a drop box is write Only for others and read only for the owner of the Dropbox (the user in this case). So If you are looking to have a shared folder then try creating a new folder in your user's public folder or on your Macintosh HD and share it. Let us know if you find problems with this approach. 

My understanding of the question is you are looking for a common folder between your MBP and iMac and not about applying proper share permissions on the DropBox folder.