How to write user logic to control SOPC DMA controller?

They do tell it, in the avalon specification (http://www.altera.com/literature/manual/mnl_avalon_spec_1_3.pdf). On pages 3-22 to 3-26 you will see the different transfer types with time diagrams. If you look at page 3-21 you will see that the waitrequest message is indeed required on an Avalon Master interface.