If you mean mounting the floppy from inside dosbox try
"mount a /mnt/floppy -t floppy" replacing /mnt/floppy with whatever the mountpoint is for your floppy disc.
If you're not sure what the mountpoint is try looking at /etc/fstab for a /dev/fdX.
On my computer it's something like
"/dev/fd0 /mnt/floppy auto noauto,owner,kudzu 0 0"
with the second item being the mountpoint.
If you mean mounting from Linux you should just have to type
"mount /mnt/floppy" using the mountpoint same as above.
Or in Redhat I can mount it by using the right-mouse click on any point of the screen going to "Disks" then clicking on "Floppy".