To use the ROMs openAOS provides you, you must change how your Archos device boots.
Installing a boot-menu will allow you to so to boot other OSes:

The above guides provides information on how to install the boot menu, but also tells you how to install an OS.
Now you can enjoy using your dual-boot install: as an example, you can boot Android and Debian on the same device!
If you run into problems, you should check if we know them already and report them if necessary.
If further help is required, you can seek help on  #openpma on (you'll need an  IRC client).