It probably is not formatted and so is not mounted. Format it in Windows
(easier) or there is a format utillity that you can install called GNOME
Format. I don't think that it is part of the default installation.
Formatting in Linux is harder to do. It is usually specific to the device
that you are using. There is no swiss army knife utility and you do not use
the file manager. You format hard disks and usb keys with GParted. You
format CD and DVD /RWs with the burner application. Floppies seem to have
gone the way of the do-do bird. However, you can install utilities and use
the commandline to do many things and floppies are still supported by many
of them. I do not know if the partition manager can handle them.

See: https://help.ubuntu.com/7.04/user-guide/C/gosnautilus-460.html

It says that you can use the file manager to do it, but I have never tried
because I rarely use Nautilus and don't have a floppy drive. You cannot
format other media as far as I know in the same way. I don't think that
other file managers (Dolphin, Thunar) have this capability either. It also
could be dated information or you could be using a simplified version of
Nautilus such as Nautilus Elementary, so it might not work. It is worth a
try.

Here is some commandline things to try to mount and format floppies:
www.cyberciti.biz/.../howto-linux-format-floppy-disk-zip-disk-create-vfat\
-ext3-filesystem/