Internal SharePoint site is not working through VPN for external computers.

I have consultants trying to access our internal sharepoint 2010 site through our VPN. Whenever accessing the site from non-company computer through the VPN after setting up domain access for those users, they cannot gain access to the internal SP 2010 site. When I or another employee accesses the SP 2010 site through the VPN from a company computer, the site loads. One consultant is using a Mac and the other is using a PC with compatible browsers. The SP 2010 site is set up using Kerberos authentification. Any ideas?