I have been unable to determine the current requirements for APEC Business Traveler cardholders to enter the Kingdom. I am an US citizen, presently in the US. Can anyone point me in the right direction?

I do not believe the possession of an APEC card makes any difference to the requirements for entry. You should check with the Thai embassy in the US to see if you qualify under any of the categories allowed to travel to Thailand and, if so, the precise process to follow in order to get the Certificate of Entry.

This from the Thai embassy in Washington DC's website.

"Due to a high number of infection rate, the APEC card holders and the applicants who are currently in the USA and wish to obtain Special Tourist Visa (STV) do not fall under the categories that could enter Thailand at the moment, please check back regularly for any updates."

Source: https://thaiembdc.org/visas/

APEC Card entry is permitted from low risk Covid countries only, e.g. Japan, HK. You still need a COE and all the necessary paperwork along with the 15 days quarantine. Currently applying for business trip using APEC.