Then they must've set that IPX tunneling, since TEN is quite dead these days.
If you're really into it, you need to have some basic idea of how to work with the xp/vista console, since you will be starting everything manually. First of all look for the dosbox.conf file wherever Dbox is installed, then look for a line that reads
IPX=false
change that to
IPX=true
You should start dosbox, then mount the drive you have Duke3d installed in with:
mount (drive letter) (location where duke is installed)
so, for a duke installation in drive D under a folder called Duke3d that line should read something like this
mount c d:\duke3d
note the backslash there, folders are named like that. After mounting, you need to switch to your recently created virtual drive, so put c: and hit enter, if you did mount it in C that is :P
then you should start ipx simply by typing:
ipxnet startserver
that should give you a working ipx tunneling started message.
In order to get your friends to play with you, you need them to connect to your IPX "server", they should do all that we did right here, but type
ipxnet connect [your ip]
then it's only a matter of setting up a net game from the setup, in the "network game" option.
I hope you can pull this off, let me know how it went!