How do I read or send a message? (BlitzIn)

Go to your Main Console. (Menu above: Window > Main Console)

Reading your messages:  
Type on the command line 'messages'.

Sending a message:
Type on the command line 'message <username> <message>' e.g. 'message Eendje Hi, I like your homepage!'.

Clearing a message:
You can delete a specific message with 'clearmessage 8' and you can delete a range with 'clearmessages 4-10'. You can delete them all with 'clearmessages *' You can delete all messages from Johnny with 'clearmessages johnny'

More information in "help messages"