Hi guys,
I would be grateful if you could help me in/on this matter.
Here's another opinion.
'With' and 'in' both seem fine to me.
If you help me 'in' this matter, it suggests a bit more to me that you are going to get personally involved, eg perhaps dealing with other people.
With 'with', it sounds like you might possibly deal with me and no-one else.
'On' sounds a little bit more odd, or at least uncommon, to me.
Clive ![Smile [:)]](/emoticons/emotion-1.gif)