Just to update, I eventually opened the package, but not sure why that was. I deleted the project, and opened the package in a different part of the computer, and this time it worked. It had previously been in my dropbox linked folder, and I wonder if this was the problem? 

Check the specified path. Most probably you will find the a *.sdlxliff file, in which name a number has been added between the primary file name extension and the extension sdlxlif. You can compare the file name in the target language folder and the source language folder. The name from the source language folder can be simply copied to overwrite the name in the target folder. After doing so you should be able to open the file.