First issue, that's a 3/16" compression fitting. It's common to use as a cable ferrule when using brake line as the conduit. This doesn't necessarily mean it won't work if that black conduit is a bit thinner than typical brake conduit. First, unscrew the rear "nut" fromnthe fitting, and check to make sure that the compression ferrule is on that conduit. If not, you can find another ferrule at your local hardware store. If you cannot find the ferrule alone, buy another 3/16"x1/8NPT fitting, they aren't that expensive ($2-3 TOPS).
If the ferrule is there, make sure that the "nut" is on there tight. If it STILL won't tighten, you can apply a piece of electrical tape to take up the slack.
If that doesn't work (I doubt it will have to go this far though), you can try taking that black conduit (or a piece if you can afford to cut an inch or two off the length) back to the hardware store and finding a smaller fitting.
If you need a picture of that compression fitting, and the ferrule inside, just to make sure you can visually identify the parts, let me know.
Your other issue, any of those devices I have messed with are intended for a 7mm (IIRC) cable, a common size for karting. If the wire isn't frayed when you try to insert it, you may have a larger bicycle brake cable. Cutting a couple strands shouldn't hurt anything, as the throttle linkage shouldn't present a great amount of resistance. Keep any potential future fraying in check (maybe with a piece of shrink wrap), you should be good to go.
Nothing to do with the cable...
Which is precisely why I threw my 2 cents worth in...
