Montana chooses delegates at the State Convention (with a non-binding primary thrown in in June just for extra fun or attention). The non-binding, basically meaningless straw-poll will quite likely be all the MSM reports on and uses to determine the color to fill in Montana on their map, as shown here
http://www.cnn.com/ELECTION/2012/calendar.html where CNN lists Montana's primary as if it were an actual primary like the ones in South Dakota, California, etc. also happening on June 5th, and completely fails to mention at all the actual event where delegates are chosen: the state convention June 14th-16th.
So, for the real and actual delegate selection process there appear to be no qualifications nor requirements whatsoever for a Presidential candidate.
For the staged show-primary, the qualifications are as follows:
*In order to qualify for the presidential primary ballot, partisan candidates
must submit a declaration and oath. Additionally, candidates not eligible
under the Presidential Primary Matching Payment Account Act must be
nominated on petitions with the verified signatures of at least 500 qualified
electors.
--
http://sos.mt.gov/elections/Archive..._Candidate_Declaration,_Oath_and_Petition.pdf
I don't know if Paul qualifies for matching funds (of course he'd refuse them if he did). Anyway, worst-case scenario 500 signatures are required. No mention of a deadline.