The Meta Keyword Tag used to be an important tag, back in the olden days of the internet. Then in about 1997, everyone collectively decided to keyword spam the attribute and now almost all search engines disregard it.
In fact, that isn't true at all. Most search engines disregard it as far as good influence goes, but they will still see spammed keyword tags as a negative attribute, so there is really no upside to be had from Meta Keyword Tags. You will derive no benefit if you use them properly, but will still be penalized if you abuse them.
This being the case, why don't you include each of your relevant keywords once each and let that be it.
Remember: Google will often use your Meta Keyword Tag to penalize you, but not to reward you so tread carefully.
Good SEO starts with a well written Meta Keyword Tag, put in some thought here and you will avoid tears later!.