Using a Different Itunes Library

Hi there, I've been using an iTunes library on an external drive to store and play music. No problem until now. I buy the Things app and want to run it from my Macbook. Itunes opens and the new app appears in my Library. But it won't open. The file chain is also here but the new file won't open from here either:
HD-USERS-DUNCAN-MUSIC-ITUNES MEDIA-MOBILE APPLICATIONS-THINGS 2.8.3ipa
It must be a simple error but any experience appreciated.
10.10.5 itunes 12.3.2

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