installation not ok

What message exactly do you get?

See README.txt:

This module integrates the Plupload library (available from http://plupload.com)
with Drupal forms. To install the Plupload library:

1. Download it (version 1.5.1.1 or later) from http://plupload.com.
2. Unzip it into sites/all/libraries, so that there's a
sites/all/libraries/plupload/js/plupload.full.js file, in addition to the
other files included in the library.
3. Remove "examples" folder from libraries folder as it could constitute a
security risk to your site. See http://drupal.org/node/1895328 and
http://drupal.org/node/1189632 for more info.

If you would like to use an alternate library location, you can install the
http://drupal.org/project/libraries module and/or add

$conf['plupload_library_path'] = PATH/TO/PLUPLOAD;

to your settings.php file.

Another thing that might be handy: http://drupal.org/project/module_instructions

try renaming 'Libraries' to 'libraries'

Great!