Enabling the asset management functionality for your helpdesk can only be done by an administrator.
If you are an administrator, go to the the admin settings and, select the add-ons sub menu: this will direct you to the add-ons page. You will then be able to select the asset management option and select one of the subscription plans. Once you have subscribed, the asset management functionality will be available for your helpdesk site and you will be able to start creating and use assets.