I remember correctly having a siscution on this not that long after the 727 v2.4 was released and we couldn't add the winglets to the 727 (not sure if it was all of them or just the Freighters).
I think we determined that it is a bug in v2.4.

Check and see if the model you added the winglets to, has the line:

ui_wglts=1

added (in the aircraft.cfg) and if you can't see them on the model itself, it doesn't work. :(