ok - In astro, when you click the Up directory Icon all the way until it says
acct
app-cache
cache
...
Scroll until you see sdcard
Select sdcard
you should see the PM31IMG.zip file there.
If it's not there then copy it there and try again.
EDIT/UPDATE: If that does not work. Connect your phone to your computer as a hard drive, and check that the SD Card drive is FAT32. (R click -> Properties on the SD card drive) If it is not formated FAT32, then copy everything off the SD card, reformat the SD card in FAT 32, copy everything back and try again through HBOOT.