OptiFDTD, as with many products is unable to offer “forwards compatibility” as in older versions being able to open design files from newer ones. There are updates to the products that propagate to the design files that just did not exist when the older versions were available. one option you would have would be to generate a layout script for your design and use that script within the older product. You will still need to create your materials and profiles but most of the structures should be supported by the older product.
When in the scripting tab you are looking for the icon circled in red in the attached image.
Scott