background in SWMM5

[asadiielts]

dear sirs,

i encounterd to problem related to background in SWMM5.
i want to model links according to my city map which is in DWG format(autocad).
manual says"with exporting to metafile(wmf)" it doesnt loose its scale and lenght ,but in my case it loose its scale&lenght.

i would be very gratful, if any one could tell me how i can able to insert my map as same as is in autocad.(sam as original file)

best regards