Mike,

Did you try the jpilot manual?  http://www.jpilot.org/manual.html

This is from the manual based on a quick google search of the words "jpilot
user name" (google is your Linux friend!):

"Sync
The sync button will sync four the main applications and any plugins that are
installed.

If you get warnings about the palm having a different userID or a different
username than the pilot that was last synced:

Every palm has a username and userid.  These can be set by using install-user
from the pilot-link set of tools.  If you changed the name or ID and it is the
same palm then you can go ahead and safely sync.  If it is truly a different
palm then you can still sync, just beware that any records pending
modify/delete, etc. will try to be modified in the new palm.  You can always
remove the .pc3 files to prevent these.  You can have multiple palms under the
same user by using the JPILOT_HOME envirenment variable.

If you get warnings about the palm having a NULL userID:

J-Pilot cannot sync with this palm because it looks as though it has been
hard-reset.  If it has been hard-reset (cleared) DO NOT sync it unless you
want to lose data.  You should use pilot-xfer to restore the palm and then
once the data is restored use install-user to create a username/ID on the palm
and then sync. 

If your palm has not been reset, but maybe just has always had a NULL userID
because you never used the Windows or Mac desktop, then good for you!  Just
use install-user to add a username/ID and sync away.  i.e.: install-user bob
1234
You can type install-user on the command line for instructions."

Did you try this?

Alan