A custom recovery is not an app. It's like a slimmed down OS. All android phones have a recovery system. The stock recovery does not have the ability to make a nandroid backup, nor will you be able to use it to flash anything but a perfectly stock update on a perfectly stock system; that is why your update is failing. On your phone, you can manually boot into the recovery system by powering the phone off, then slide out the keyboard, then press and hold the "x" key while powering the phone back on. But that is just some info about the recovery system.
If your phone still boots up normally, I think the easiest way to install a custom recovery and be ready to flash custom ROM's would be to install ROM Manager (free) from the market/play store. After you've installed it and open it for the first time, the top oiption in the menu is to flash ClockWorkMod Recovery. Select that option. When it asks for Superuser permissions, make sure the 'always' box is checked, then hit OK and the app should download CWMRecovery, flash it and if all goes well, you will have a custom recovery; however on a stock rooted system, the custom recovery doesn't always "stick".
I think the ultimate thing for you to do is to go to the "Motorola A855 Guides" link in my signature because I have a plethora of info there that will further help you to get some basic understanding.
good luck