There is now an InDesign-CC compatible in5 installer available.
Be sure you have the latest version of InDesign
Before installing in5, make sure you have the latest version of InDesign CC. Try updating InDesign the Adobe Creative Cloud Desktop (which itself will have to have update 18.104.22.168 or newer).
Or more directly, use Help > Updates… within InDesign CC.
Check the InDesign version by going to the InDesign menu (or Edit in Windows) and choose About Adobe InDesign…
After the update, your InDesign version should be at least 9.1. At that point, you can use the in5_cc.zxp installer.
Hopefully the update will just appear for you in the Adobe Creative Cloud App. For me, I had to quit the app, delete the file listed below, then restart Creative Cloud for the update to appear (but I was trying to test this immediately after release).
<Startup Disk>/Library/Application Support/Adobe/AAMUpdaterInventory/1.0/AdobeUpdaterAdminPrefs.dat
If you do not have the latest version of InDesign CC the in5 dialog will appear blank.