The contact form solution pack is not an extension.
There is no MXP file to install.
solution packs are pre built php applications that where created using the extensions.
to use the contact form solution pack, you need to edit the settings in the WA_Globals/WAGlobal.php file for your server environment, then upload the contact form solution pack files to your host.
Once everything is uploaded, browse to the contactus.php page, fill in the form and the information will be emailed to the address you specify in the WA_Globals.php file.
more specific details can be found in the Getting Started guide on the contact form solution pack support page: