I am under the impression that your personal trainer certification also has to be through NASM to apply for their Master Trainer program. It will be best if you call them 1-800-460-6276 and ask your question there.
I just completed the NASM Master Trainer program this past month. In order to become a NASM Master Trainer you first must hold a valid NASM CPT for 2-3 years. You need to also have the PES, CES and one other speciality cert (GFS, WLS, FNS etc…). You then apply to NASM and get accepted to take the program. The Master Trainer program consist of an online portion (which you have to complete in 5 weeks) and then a live practical in which you have to perform a fitness evaluation and train another Master trainer candidate using the OPT model. Needless to say, you need to have your anatomy, biomechanics, and the OPT model knowledge down pact to pass. The program is challenging in nature, but its probably the only legitimate Master trainer program in the fitness industry that I have seen. Other fitness certifications (ISSA, NESTA etc…) just make you buy a bunch of their certs and then just give you the title “Master Trainer”. In the NASM Master Trainer program; you have to work and earn the title based on your knowledge and experience. NASM is very widely known and respected in the fitness industry. I recommend looking into it. Hope this info helps you out.