Then make sure when you build your kernel you have usbserail AND ftdi_sio (assuming your device uses ftdi serial) built as modules. Make sur eyou actually build and install the modules with make modules modules_install. Reboot into the new kernel and update-modules (your distribution might call this something different).
At that point you need to find the vendor id and product id of the device you are trying to connect to. If you have lsb or usbview you can get that information. Use it for the next line:
modprobe ftdi_sio product=<productid> vendor=<vendoriid>
For example to talk from my linux box to the plug I have
modprobe ftdi_sio product=0x9e8f vendor=0x9e88
If you are using udev, you can make a rule for pluggin and unpluggin the device like this:
/etc/udev/rules/d/10-mydevice
SYSFS{idProduct}=="9e8f",SYSFS{idVendor}=="9e88",ACTION=="add",RUN+="/sbin/modpr
obe -q ftdi-sio product=0x9e8f vendor=0x9e88",SYMLINK+"USB[0-1]"
ENV{PRODUCT}=="9e88/9e8f/500",ACTION=="remove",RUN+="/sbin/rmmod ftdi_sio usbser
ial"
You'll have to get the ENV{PRODUCT} number for your device from the command
udevinfo -a -p $(udevinfo -q path -n /dev/ttyUSB1)
once you've modprobed ftdi_sio to set up the remove rule.