Hi Antonio,
does the path you entered sure exists?
The path is always extended with "\bin".
If your LAStools location entered in the settings is "D:\LasTools" your exe files have to be found in "D:\LasTools\bin".
If you can not fix it please provide some screenshots from
(a) your installation dir
(b) your QGIS settings page and
(c) the tool log output.
Cheers,
Jochen @rapidlasso