To sync Stacks data with Dropbox service, first you need to download Dropbox from here

After you've have configured Dropbox folder on your device, assuming you currently have Stacks file saved on a local folder, you just need to move your current Workplace folder from the local folder to a Dropbox folder following the Moving a workspace guide.

After the files have been synced, to access the workspace in the other pc/mac you just need to install Stacks and follow the guide to add a new workspace, Adding a new workspace locating the Dropbox folder you have just synced.