Mercenary, or Private Military Companies (PMC's) are fluid, rapidly changing businesses with high volumes of operators and employees flowing in and out constantly due to the contracting nature of the business. There is almost always an open spot to fill for these companies, and hopefully this article can steer you in the right direction.
An important factor to remember is that many of these jobs are filled by word-of-mouth. One former soldier may call up an old buddy of his and see if he wants to earn some extra money, for instance. And since military experience is all but essential for this line of work, it is likely you will know some PMC employees if you are a veteran. If this is the case, by far the most successful way of securing a job is simply by calling around and asking friends if they can help set you up with a meeting. Between them personally vouching for you and setting you up with an interview, you can almost be guaranteed a job with the company if you are qualified. If this isn't feasible, there are many methods of finding companies interested in hiring you.
The next thing you should consider doing is to look for local job fairs in your local and regional area. Many of the top-level PMC's, such as Blackwater and Raytheon, hold job fairs to recruit qualified operators for critical openings. They will have short seminars about what they do at their company, provide job information, and will take questions in addition to handing out applications and other informational literature about their company. Having a good first impression at one of these events could yield serious offers for you.
Lastly, looking up the companies and calling their personnel directors to directly inquire also has serious potential. The overwhelming majority of employees at these companies are veterans as opposed to typical office workers, and being talked to directly and openly is something that they certainly would value. Being forthright about your talents and qualifications without bragging or being too modest will certainly earn you respect, as well.
In conclusion, Private Military Companies are very different from almost all other companies, and getting hired is an entirely different process too. Your qualifications on paper are almost unimportant compared to other skills you may have, but don't let the unorthodoxy of these companies deter you from seeking a profitable and important occupation.