Where should I install the LESSPHP compiler?
In the Less Processor Module page it says:


  Requirements 
  
  Libraries API 
  
  less.php and unpack it so that
  'lessc.inc.php' is located at
  'sites/all/libraries/lessphp/lessc.inc.php'.


Which means: 

1) you need to download and enable the Libraires API module 

2) Then you need to download Less.php &lt;-- click here to download it

3) Open the zip file, and grab the lessc.inc.php file only 

4) Next you'll need to create some folders in your web directory (create a libraries folder inside the all folder, and a lessphp folder inside the libraries folder) so it looks like this

sites/all/libraries/lessphp/


5) Upload the lessc.inc.php file to the lessphp folder.