Here is what Chatr lists on their website:
If you have an iPhone/iPad
For some IOS version, you can view the APN settings on your device in Settings > Cellular > Cellular Data Network and change it to chatrweb.apn
For iOS5 or later please click www.chatrmobile.com/setmyiphone from the internet browser of your iPhone/iPad to get the settings automatically pushed to your device.
If you have another type of phone (Android, Symbian, etc.)
For chatr data:
The data APN is chatrweb.apn
For chatr MMS settings:
The MMS APN is chatrweb.apn
The MMS URL is http://mms.chatrwireless.com
The MMS IP address is 205.151.11.11
The MMS port is 8080