UPDATE - I got it working, the problem was the zip file contained the file structure Oquonie/Oquonie.pdx/<all the subfolders & files>, but the playdate website expects the Oquonie.pdx to be the root folder inside that zip file (i.e, just Oquonie.pdx/<all the subfolders & files>). Once I zipped up the Oquonie.pdx folder by itself the upload worked no problem