OptiFDTD versions above and including 12.0 are strictly 64 bit. Are you sure you have 32 bit software?
Unfortunately, I don’t have access to the 32 bit example file, but you may be able to find it in your example folder, which can be found at “Documents\OptiFDTD 12 Samples\3D_samples\plasmonics”. Another option is to download a 7 day evaluation copy of OptiFDTD, which will give you access to the most up-to-date version.