Here's the macro level view for park indices (home runs)...
A park index of exactly 100 is a neutral park -- it has no effect on this particular stat. An index above 100 favors the statistic that it is easier to hit home runs in said ballpark. The opposite is also true - if the index is below 100, it is more difficult to hit home runs in said ballpark.
In 2010, Wrigley Field had a park index of 110. Meaning it's 10% easier to hit HRs in Wrigley than in a neutral park. The Cubs hit 74 home runs in home games while their opponents hit 87. Miller Park had a park index of 123, Busch Stadium had an index of 77, and Great American Ballpark had an index of 114.
U.S. Cellular Field had a park index of 157. WOWZA! It was 57% easier to hit a HR in