the smb.conf file is hidden to a normal user. you need telnet access to be able to see it.
Just a clarification that may not be obvious to the new user. If you just write to /etc/samba/smb.conf then the changes will be lost with each power-down/reboot.
You need to write the changes to the version stored in flash (which if done improperly could make your machine inoperable. If you are using Fonz's fun_plug you may want to look at /ffp/sbin/store-passwd.sh for an example of how to write (relatively safely) to the flash memory).
I haven't looked at this extensively, but it appears based on a quick look at /etc/rc.sh, that to make the changes permanent you will want to first edit /etc/samba/smb.default and the write those changes to the corresponding files in /dev/mtdblock0 (and maybe also /dev/mtdblock1) -- look at either /etc/rc.sh or /ffp/sbin/store-passwd.sh for examples on how to do this.
I believe that what happens is that the compiled program /usr/bin/smbcom (which is run on boot from rc.sh) concatenates samba.default (which contains the system-level config parameters) with /mnt/HD_a4/.systemfile/.smb.ses [note may not be HD_a4 on your system] (which contains the user/group samba share details). The concatenated result (minus any commented lines) is then written out to /etc/samba/smb.conf. Then rc.sh launched the smbd/nmbd daemons.
The bottom line is that if you want the changes to stick, you will likely need to make changes to the flash version of smb.default. Also, if you change the smb.default file then the changes won't take affect until you reboot or alternatively you could probably kill the smbd/nmbd daemons, re-run smbcom and then restart the daemons. Similarly, if you edit /etc/samba/smb.conf directly the changes won't take effect until you kill smb/nmbd and then restart them (or alternatively you could probably just send a SIGHUP to the smbd process)
AGAIN DISCLAIMER - I CAN'T PROMISE THIS WILL WORK, I AM ONLY GIVING SOME POINTERS. THIS PROBABLY WILL VOID YOUR WARRANTY AND IT COULD BRICK YOUR MACHINE IF I AM WRONG OR IF YOU MAKE A MISTAKE.