i use the fx-991 ms model....
and in tht...for these conversions...
press mode...twice...press 3 and select base option...
u see a "d" sign....tht is decimal...
so if u want to convert say (100)d to binary(or hex)...type 100 and press equal to...now for binary...press tht log button which has BIN written over it in green.......u get the conversion....tht is 100 in binary.....similarly for hex use the " ^ " button and so on....