You don't create mattes in Twixtor, you can either use them or use the mask geometry to help twixtor (then the matte render of mask in AE UI must be set to OFF/None so it does not render the mask to the alpha channel).
It's better if possible to send a small sequence project to techsupport at revisionfx dot com so we can help you out.
yeah, are you seeing this on your background layer? I presume this is because you need a matte with antialiased edges, which works great for the foreground. So when the matte is used to cull out the foreground when calculated the background layer you still see an edge. Does this sound right?
Two answers: you can use InvMatteShrink (see manual), which grows the foreground matte by the specifiied amount (in pixels) to allow for edges that need an antialised-edge matte for the foreground. This way you can probably remove much of this extra line you are seeing.
As Pierre said, if that doesn't work, send us the project (I presume it's for the footage you sent the other day?).
Also, we had a major website/email server failure yesterday and are still recovering... which is why we have been slow in responding.