Just apply like everyone else.
See here:
http://www.mstp.northwestern.edu/ad_info.htm
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International students are eligilbe to apply to the MSTP if they have completed at least 3 years of full-time study (undergraduate, graduate, or post-baccalaureat) at an accredited college or uiversity in the US or Candada.
Students already enrolled in several schools at Northwestern University are also eligible to apply the MSTP and should contact the MSTP Office for specific instructions.•
The application cycle begins in June, but you should prepare everything starting now (start asking for recommendations and writing personal statements) and have it done by May.
There actually might be a disadvantage in applying to the same school from which are you coming. MD/PhD programs act more academically and it is generally a good idea to visit other institutions. Northwestern is a great school but there are many others.
If you want to stay in Chicago, the Pritzker School of Medicine and the University of Chicago has an excellent MD/PhD program that is as good or even better than Northwestern's (U of C students tend to think Northwestern is overrated).
There is also the University of Illinois, which is the largest medical school in the country. Its Chicago and Urbana campuses have excellent research facilities as well depending on what you want to do.
Keep in mind, however, that as an international student it will be more difficult to obtain funding. You are not eligible for MSTP funding which will make getting into Pritzker or Feinberg more difficult. U of I has separate funding sources which you may be eligible for.
