Below are the steps to troubleshooting your cards.
Check Driver Modules
Check Devices and Permissions
There are now two dtv programs for checking signals dtvsignal and dtvsnr. If you are using kernel 2.6.18 then you must use dtvsnr as the driver in the 2.6.18 kernel will not return a signal level for the HD-5500 card only the SNR level.
where ## is the channel number of your ATSC station you want to recieve.