You need the Wedding Collection File for the Arch, this (when you unzip) goes into the collection file. The other the Community Lot Wedding File (unzipped), goes into your downloads. You will find the wedding arch in build mode in the collection files.
Have you tried clicking on a mirror for change appearance? sometimes that works to fix blue flashing problems. Also Have you tried the sims 2 hack conflict detection utility? to check for a possible mod conflict.