Hey mate,
In WHM on the left hand Nav Bar, Select 'Software'->'Module Installers'
Here you can select 'Manage' where it says PHP Pecl, after a few seconds you will be redirected, once this page loads you will be able to search for mailparse and select 'Install'
If mail parse is installed than you will have to go to php settings and activate it,if it is not installed than you will have to install mailparse WHM-->software-->module installers-->PECL and click show avaiilable modules or just search for mail parse in search field.
You can of course install it through SSH also if you preffer.