Not sure what to say, it should work fine. But if you're doing everything correctly there's something wrong with the PS3 or the install. Did you try redownloading the Strato mod incase the initial download was corrupted/incomplete?
One final suggestion if you know a bit about modding, you could copy the PDIFS folder with just Spec II installed to a PC, then extract the files. Do the same with the PDIFS folder for the Strato Mod seperately, and copy the extracted files across then recompress the PDIPFS folder and copy it back to the PS3.
Those are the final things I can suggest, failing these, there's not much else I can advise I'm afraid.