Mhl is not compatible with any new Nexus devices: it's used by Samsung only so it will work with the Galaxy Nexus.
Slimport adaptors are compatible with Nexus 4, 5 and 7 (2013 only), so its normal that you can't get your 2012 nexus 7 to work.
Instead, it should be perfectly working with a Nexus 5, since no previous configuration is required.
In my case, the first time I connected my N5 to my HDTV via Slimport adaptor it took about ten seconds to see the home screen of my phone, but since then, it has always worked just after connecting it.
Inviato dal mio Nexus 5 utilizzando Tapatalk
Google Nexus 5 16GB White - Android L Developer Preview - Rooted
Google Nexus 7 32GB 3G - Full Stock
HTC Desire HD - taktiK - Android 4.4