Importing libraries in Processing IDE
FreeIMU is not a Processing library by itself but an Arduino one. So including FreeIMU in a Processing project won't work because it is made to access specific hardware and collect data from specific gyroscope and accelerometer chips.

To visualize graphically data on a computer (PC) with Processing, you have to communicate with the Arduino where FreeIMU is installed, to collect the data.

The FreeIMU library package, available here, have two Processing examples. If you look at the code provided you'll have your answer.

FreeIMU_yaw_pitch_roll Processing example :

Visualize orientation information from a FreeIMU device

INSTRUCTIONS: 
This program has to be run when you have the FreeIMU_serial
program running on your Arduino and the Arduino connected to your PC.