As of the 0.5.3 FireTray update, the check marks for selecting "Included accounts" have become invisible. It is still possible to toggle them by clicking blindly (the changing tray icon verifies this).
Unfortunately reverting to 0.5.1 after uninstalling 0.5.3 does not appear to restore the check marks.
This is on 64bit Linux Mint 17 with Mate, (also confirmed on 32bit Linux Mint 17 with Mate).
See attached image.
Reverting to 0.4.8 restores the check marks.
It happens to me too. Linux Mint 17.1 Cinnamon.
Thanks for reporting. I'll look into this problem soon.
Fixed in 972f1c6. I'll provide an updated version on AMO ASAP.
In the meantime - where is the checkbox? Left, center, right?
It's centered in the Included column.
If you don't want to wait for the AMO release, you can easily build it yourself