That will probably be the BIOS files location stored in the Alitirra.ini
the BIOS files must also be in the root of the emulator folder, and named exactly the same as they are in the script.
i would use the script and emulator setup from Atari800 v13.
*Altirra has beed updated many times since the demobase release.
current beta.
http://www.virtualdub.org/beta/Altirra-3.10-test34.zip