Oh, sorry about that. To figure a park factor, you just take the average total runs scored in a team's home games and divide it by the average total runs scored in that team's away games (both the team itself and its opponent). Because both home and away teams are taken into account, it controls for home-field advantage. The higher the number, the more of a "hitter's park" it's considered to be. If, for example, there are 10 runs scored on average in a team's home games and 9 runs scored on average in its road games, the park factor is 10/9 = 1.111, which means the home field is more or less a hitter's park. There's some randomness and variability from year to year, but you can get a pretty good picture.
ESPN has the numbers
here. The imaginary field in my example would have ranked 5th overall in MLB.
They also do a similar exercise for home runs, not just total runs, to give a sense of how easy it is to hit out of a particular park. On the ESPN page you can sort it by home runs (or by hits, doubles, triples, or walks -- and a park can, in fact, affect walks, with wind patterns, glare from the sun, etc. etc.).
Anyway, my point is: if you go through the list, either for total runs or home runs, the AL and the NL come out pretty even overall.
Of course, since this thread is about Johjima, I would point out that it's quite possible he will benefit from going against NL pitchers, though not necessarily from hitting in its parks.
