Downgrade El Capitan to Mountain Lion Issues

I have a Late Model 24" 2008 iMac 3.06 GHz Intel Core 2 Duo Processor with 2 GB 800 MHz DDR2 SDRAM that currently has El Capitan. I've had it since 2008, too. I've done all of the OS upgrades and now I realize it has screwed me as far as my outdated audio interface is concerned (Alesis iO 26 Firewire). I have another mac laptop that I use on a regular basis.

I'm a crappy musician that likes Garageband for its simplicity. It doesn't recognize my interface(AI) anymore. After some consultation and research, I realize that Mountain Lion(ML) is the latest OS to work with my AI.

I purchased ML from the app store and downloaded it with the intent to wipe my HD clean and do a clean install of ML.

I have a 64GB thumb drive that can't be partitioned for some reason. I erased it and copied the InstallESD.dmg on it. I rebooted this computer holding Option a bunch of times. I never get the option to boot from the thumb drive.

What am I missing? What am I doing wrong? I'm not an idiot but I don't know that much about computer stuff. The tutorials that I have found never have "every step" I have or they have "too many" steps that I don't have the option to do.

Any advice would be GREATLY appreciated! Thanks in advance.