It's an mde file (meaning compiled version of the code.) You can make mde files out of mdb files using a tool in MSAccess but you cannot edit or change the code/form in any way within an mde file. It has to be done on the mdb file first and then re-compiled into a new mde file.
Find the mdb file and then make your design change on that (if you're using an mdb file and STILL cannot make design changes, then it's MSAccess security that is preventing you from making design changes). Once you make the design changes in the mdb, make your mde for the users to run.
The question is still though: What file type are you opening (ACCDE (for MSAccess 2007), MDE, ACCDB (for MSAccess 2007), or MDB)