After you allow it to create the project with all lower case, try to Save As. It won't allow you to just fix the case and save it.
Saw that - Windows file name can contain upper and lower, but the file system is case Insensitive, so you cannot have "abc.dmd" Save-As to "ABC.dmd", because it's the same file (you can only "save" it, not "save-as" to a "different" file that is the "same" file).
So close the project. In Windows, I go and fix the case of the 3 files it has created. Now you cannot open the Fixed Filename.dmd project - "Error reading project file!"
I changed the 3 all lower case files
"bsaveas.dmd", "bsaveas.wsp", "bsaveas.xml" to
"BSaveAs.dmd", "BSaveAs.wsp", "BSaveAs.xml" using Explorer
and it opened just fine in Designer. I am using DmD 1.3. Is that they way you see the error, or do I need to do something different?