Where is the Lord Mayor of London?
(Why is 'Lord' added before 'Mayor'here? In the sentence, what is the meaning of 'Lord'?)
It's just his title. Some are referred to as simply the mayor, but in some very large cities (especially London and Dublin), the person who holds that office is referred to with that title.
http://en.wikipedia.org/wiki/Lord_Mayor_of_London
The Lord Mayor of London is not permanently located in just one place. The Lord Mayor's official residence is known as Mansion House, which is in the City of London. However, the Lord Mayor carries out various official functions in different locations throughout the year.
The term "Lord" in "Lord Mayor" signifies an honorary title. It is a traditional title given to the Mayor of London. The addition of "Lord" differentiates the Mayor of London from mayors of other cities. The Lord Mayor holds a higher ceremonial position and represents the City of London, which is the financial district, on a national and international level. This honorary title has been in use since the 12th century and is a unique feature of the City's governance.