Due to the convoluted path the code has gone through, the programming interfaces for MGR, WIMP and WIMPi bear only a superficial resemblance to each other at this point. Many of the functions directly correspond to their counterparts in the Scout winMgr interface, however. For more information on those, see in the OSKit source tree wimp/scoutdoc/{winmgr.h,wimp.tex} from the Scout distribution.
To use wimpi, you must first initialize it by calling wimpi_initialize. Once you have initialized all the windows for your application and are ready to enter the event loop, you will then need to call wimpi_main_loop with the wimpiSession variable you got from calling wimpi_initialize.
You will also need to have handlers to send input data to wimp, and to handle wimp events when they happen. These functions are set by calling the wimp_set_input_handler and wimp_set_event_handler functions.
You can also look at examples/x86/wimpirun.c for more information.