I am a U.S. Citizen living in Abu Dhabi. How do I get tourist visa for 5 day stay in Beijing?

It cannot be completed online.
You should first prepare the relevant materials including your residence permit in UAE, a filled-out application form attached with a passport-style photo, and proof of transportation and accommodation, as well as your passport and copy of its data page. Next, submit the application to the embassy of Chinas to apply for the tourist L visa.