The drug works by boosting the immune system. Specifically, LDN increases the body's production of endorphins and enkephalin. These opiods apparently have some regulating effect on the immune system (although the exact effect appears to be unknown). From what I've gathered, there are other immune therapies, such as acupuncture, that may have a similar effect. Acupuncture, for instance, likely triggers the release of endorphins as well.
The recent study conducted at Pennsylvania State University to use LDN for Crohn's therapy used 4.5mg at night before bedtime and had great results. The positive results prompted additional NIH funding to proceed with clinical trials.
Another source of info (in addition to Wikipedia) is this website: http://lowdosenaltrexone.org/. It includes information on how and where to get low-dose naltrexone. Naltrexone is already widely available, but you need to get it altered to put it in low-dose form. There are a limited number of pharmacies that do this.
Another thought that this treatment prompts is that this treatment works in direct opposition to most traditional therapies. While most prescription drugs are meant to stop the immune system (i.e. immunosuppressants like Entocort or Prednisone), this treatment is meant to boost the immune system. Which way is right?