I met Alex Manzo at Confluence Kayaks in Denver. I understand that this is his company.
The security situation in Northern Mexico is poor, so you should fly, probably to Tampico and then bus to Ciudad Valles. Ciudad Valles is the center of the Huasteca area, where most of the whitewater is located.
I am a member of an email list of Mexican sea kayakers, but some of them run rivers also, so I have some access to additional information.
I have been wanting to get down to the Huasteca for years, but the security situation has discouraged me.