Heh, no problem. Despite using RISC OS for too many years than I care to mention, Arcelite is such an immense game that I can think of little better reason to use it.
I've not actually emulated RISC OS on my Pandora, and stuff like swapping OSes about and disc images is specific to the emulator, and not a RISC OS thing, so I may not be able to help you much. RISC OS is a simple ROM-based OS though, like the old 8-bit and 16-bit systems, if that's any help, although versions from 3.5 on have a !Boot app on the disc which can be OS-version specific, but hopefully the most recent versions will work all the way back to 3.5.
In terms of getting ArcEm working on RISC OS, I assume you've downloaded 1.50 from
http://arcem.sourceforge.net/index.html#downloads.
That gives you a zip which appears to be some new packaging format I'm not familar with, but if you just want to pull ArcEm out of it it's there in Apps/Misc/!ArcEm. I'd suggest you unpack it on RISC OS rather than Linux though, as RISC OS files have intrinsic filetypes which will need to be right if the app is to work. Hopefully
SparkPlug can unpack ZIP files, but if not you can pick up the InfoZip binaries from
Ms Bazely's site. Note you'll need Sparkplug to unpack the infozip archive though, so you'll need that either way.
Hopefully that's enough to get you started. If you've any more questions, especially about the RISC OS side of things don't hesitate to post here.