Actually, I’m having the same problem. My ‘Subscribe to Download’ problem started midway from 4.1 to 4.2. But that time logging in solves my problem. But after using the offline mode due to internet problems some time after upgrading to 4.1.1, the ‘Subscribe to Download’ issue stays even after logging my account. I didn’t bother at first because I already downloaded all the free contents, but now I think this is a problem.
I tried to test some solutions on my own, and the Install button on 4.2 now appears. Try either of this:
Log-in your account to unrealengine.com, and restart your computer. Open your Launcher again.
Remember 4.1.1 update where you have to type in the “applabel=…”, I forgot already the entire word, for us to have access to the update? Remove it leaving only “C:\Program Files\Unreal Engine\Launcher\Engine\Binaries\Win64\UnrealEngineLauncher.exe”, then, open your Launcher.
On my end though, I restarted my computer after logging in and editing the property of the shortcut so I’m not sure which one works but I’m guessing it’s the second option since I did the first a lot of times already.
This works on me at least, since I can see the Install button for 4.2 now.