Depending on your transmitter, use ch. 3+7 (7-channel receiver) or 7+8 (8-channel receiver) for left and right throttle. See this link to set up your mixes (use PMIX):

http://www.futabarc.com/faq/faq-9c-q534.html

http://www.futabarc.com/faq/faq-9c-q511.html

Hope this helps...