Cannot get IM+ Messaging to work!

Ok...I am leaving for Japan in 2 days and I'm getting frantic!
I want to be able to text using my data cause Int'l. text costs 0.50 ea. I've tried to email using the cell#@vtext.com...it worked once and after that the person receiving the sms can't reply.
I then bought 2 copies of IM+ Messenger, for my daughter and me, mySpace and FaceBook chat works fine. I got both of us Yahoo messenger because IM+ says that you can send sms text using your Yahoo messenger. I can send sms text through Yahoo fine but when the person I've just texted replies I don't get anything! Am I supposed to get a reply through email or that IM+ app? I've tried this with both phones and it's the same. :confused::confused:
Is there a setting I am missing???
PLEASE HELP!!!:)