An instant message (IM) in Second Life can be sent via a resident's profile, by right-clicking their avatar and selecting "IM..." from the pie menu, from the friends list. If the resident is already a friend, open the Communicate Window, select their name from the Contacts/Friends list, and push the IM/Call button.
When an instant message is received, the IM button in the toolbar will blink and an "IM Received" button will appear at the bottom of the screen. Clicking either of those buttons will bring up the Communicate Window.
