You might need to give a little more info as to the exact M8N board you have and the version of operating system ( XP ,VISTA ?)your using to get sufficient help.
There might be something turned off in the BIOS for the PCI slots that's stopping them from working.
Are they new TV cards or from an old machine . Are you sure they are working cards.
Have you got another pci card that could be put into the slot to see if that works like a cheap sound card or a network card and then see if the operating system tries to detect the new hardware. This would tell you if the PCI slots are working ok.
Also try installing the card ( with pc turned off of course) reboot, then go to settings, control panel, add new hardware. Follow the wizard to see if it detects it . If not try the select device from a drop down list and then add the driver for the card.
You could also right click the my computer icon , select properties, hardware, device manager and look to see if you have a yellow question mark in the list of devices when you have the TV card fitted . This would show that the pc has detected it but you just need to get the operating system to accept / load the driver for it. Click the question mark and try to add the driver for it ( i'm presuming you have it on file / disc or downloaded to the pc somewhere)
Hope this helps in some way and sorry if some of the info is a little basic.