The latest versions of Inkscape have the plot extension built in. You can find it under Extensions>Export>Plot in 0.91. It only supports a few languages, HPGL being one. It only works with a com/serial port and will not work on cutters that use a LPT port.
The com port must be functioning, cables, bios, irq's, usb adapters and all. It is not as robust as commercial software. As it stands there are limited open sources options for cutters. There are variations like TuxPlot and Inkcut, but those are probably going to fail if the Plot Extension fails.
One method of testing your environment would be to download a trial of commercial cutting software, with limited ability, such as SignCut Pro. It will only let you do a few things, but if you can not get a trial version to do anything, it says something about your set up being wrong or some equipment/cable failure. If you get a trial to work, you can often take those same settings and try them with Inkscape's Plot Extension.
Back in the day when com ports were it, the best piece of gear I had for testing if a com port was alive was a serial modem. You could use a terminal program to access the port an enter AT commands. If you got a response, the com port was alive. Somewhere along the line I lost my serial modem.