Where in North America is area code 525?

There is no area code 525 in North America (USA, Canada, etc.). It is unassigned.

Telephone country code +52 is Mexico, and numbers beginning with +52 5 are landlines in or near Mexico City.

(The plus sign means "insert your international access prefix here." From a GSM mobile phone, you can enter the number in full international format, starting with the plus sign. The most common prefix is 00, but North America (USA, Canada, etc.) uses 011, Japan uses 010, Australia uses 0011, and many other countries use different prefixes.)