"But when I have 'Russian' selected, it still shows up as grayed out - and won't let me preview it, for example"
This apparently is a good sign! Because it's a known - cosmetic really - issue that when one installs phonetic layout which was made earlier under Windows XP/Vista/Windows 7 then Preview does not work.
I do start my phonetic instruction with that note about that non-important, cosmetic issue under Windows 8, please look:
http://winrus.com/kbd\_e.htm#p1251
So it tells us (along with 419 entry you made a screen-shot of) that we successfully added phonetic layout and registered it successfully as "Russian".
- Now we only have a switch / indicator ENG / RUS issue
a) when you go to Control Panel - Languages, do you so _both_ English and Russian there?
(it's only when you click on "Russian" you get the screen you provided in your screen-shot)
If you do have English and not just Russian:
try to re-start Windows to let it 'catch up' with those changes and to present you ENG / RUS indicator
Paul