arduino 1.6.4 and average libraries problems

I am new to Arduino, so please help if you can.

The problem:
I have some codes using the mean, maximum and minimum function provided by average library. However, when I compile it, errors are:

error: 'mean' was not declared in this scope.

The solution:
Google it, I need the average library. The download link is:

The zip file is with an name 'Average-master.zip'
Based on the instruction

Sketch-> Include Library->Add .zip library, the file is located and shown in the library list successfully. Close the IDE and restart it again.

However, the same error still shows up when compiling.

Any suggestions ? Thanks in advance.

One thing forgot to mention, in the library management, says: Average-master Version unknown INSTALLED

bump. is this a hard question to answer ?