[LDCad] Problem exporting model to ldd

[Moe]

I'm making a Lego robot arm build guide in LDCad. But being a novice, I accidentally created a sub-model within the main build window. My finished model is now saved as 'LDraw Multi-Part Project' instead of as an LDraw file(*.ldr). I found my mistake after many hours of build when I finally tried to export my build guide to Lego Digital Designer, as it is the format I was expected to submit. When I try to import this file to Lego Digital Designer(LDD), LDD only looks for *.ldr files only. I want to save my sub-model as main model in LDCad with all the steps I used to create it. A search on the internet hasn't helped me find solutions. I really don't want to go back to the drawing board and spend many hours I barely have on building it again. Can anyone help?