Once you have a bootable kernel for your Explora, you'll need a file system to go along with it. The explora kernel should be able to execute a linux executable built for the PowerPC 8xx line (860, 850, 823, etc...). Executables built for the PPC 405 or other 4xx PPC cpu's probably won't work due to cache line size differences.
The simplest method is to build a file system which is included with the kernel as ram disk. I'll include one here.
The best technique for developement is to NFS mount a full file system.
At some point we'll have PCMCIA support working and we should be able to mount PCMCIA IDE flash cards as a file system also.