(Credit: Screenshot by Nicole Cozma/CNET)
Step 1: Open the Dropbox app on your Android device. If you're new to Dropbox, you might have a small tutorial you can read through (or skip).
(Credit: Screenshot by Nicole Cozma/CNET)
Step 2: Press the Menu button and then Settings. For Ice Cream Sandwich users, this will be the small three dots in the corner of the screen.
(Credit: Screenshot by Nicole Cozma/CNET)
Step 3: Scroll down to Passcode lock option and press on the check box next to it.
(Credit: Screenshot by Nicole Cozma/CNET)
Step 4: Touch Turn passcode on and enter your preferred passcode, then re-enter it for confirmation.
(Credit: Screenshot by Nicole Cozma/CNET)
(Optional) Step 5: You can also choose to Erase all Dropbox data from the device after 10 failed passcode attempts. Make sure you've picked a passcode you will not forget if you're going to select this option.
That's it! Now each time you open the Dropbox app you will be prompted for your passcode. Dropbox also does a great job of giving you the onscreen keypad to make entering the code as quick as possible.