Niall,
I took these steps to recreate your issue. I found your account. I downloaded the version of ProPack. I went into my account and deleted my version of ProPack, I deactivated any of the plugin add-ons I had so that I would only have proPack and SLP. I activated ProPack it activated as 4.6.4 SLP is also 4.6.4 I have import. I then went to the Import choose file, I uploaded a CSV file I had. It uploaded and added my locations, no problem whatsoever.
Please follow the steps exactly as outlined above.
Here are screenshots of my plugin environment. If yours is different, for instance if you have multi sites or did not deactivate and delete please let me know.