KK, redownloaded the BC resource packs to make sure that we're on the same page. Follow the following steps exactly:
1.) Extract Better Cities Resources 4-5-2 Part 1-16513.zip to a temporary folder.
2.) Extract Better Cities Resources 4-5-2 Part 2-16513.zip to that same temporary folder.
3.) Check that you now have "Better Cities Resources BSA 4.5.7z.001" and "Better Cities Resources BSA 4.5.7z.002" in said folder. .001 and .002 are used to denote that the archives have been split, and will be re-merged upon extraction. This is done to avoid the file size limitations @ Nexus I believe.
4.) Select *both* the .001 and .002 file, "right-click", and from the 7-zip option, choose "Extract Here" (technically, you only need to do it from the .001, 7-zip does the rest in finding the other parts, but I like to be paranoid
)
5.) Confirm that Better Cities Resources BSA 4.5.2.7z is the resultant file.
6.) Open OBMM and create the OMOD from step 5's file, utilizing the normal methods, importing the OMOD conversion data when prompted.
Hope that helps.
EDIT: TL;DR: read Tomlong's above post