I had this problem today, I wanted to create a backup before making some changes. WordPress Backup 2 Dropbox appears to only have a schedule setting. A lot of people complained in the reviews of the plugin about this and gave it low ratings.
But I discovered this functionality is actually there, it's just in a place nobody would ever think to look: Backup Log.
How To Backup Your WordPress Immediately with WordPress Backup 2 Dropbox:
1. Login to your WordPress Admin section.
2. Find WPB2D on the sidebar and click 'Backup Log'
3. Click 'Start Backup'
4. You're done!
It can take some time to generate the backup, but it seems to restart itself pretty well and get it done, eventually.
I hope this saves some WordPress Developers time in the future!