I have unlimited texting and picture/video texting.
Does that mean I can use Mobile IM with no charges? Or if it does cost money, how much?

This is the message I get on my phone when I want to use Mobile IM [keep in mind I have unlimited texting]:

You are about to access the Instant Messanger application. Sign-In and IM contact list udate commands sent from this application will use airtime that will be billed per your Calling Plan. All other instant messages and commands will be in the form of TXT Messages that will be billed per your Calling Plan or TXT Messanging subscription.
[accept] [decline]

Thank you!!!