when somebody's profile viewing blinked in green 'on line' what does it mean? and how can be chat with him/her?

HOW DO I IM?
First, you must have YOUR 'Instant Messaging'
turned ON:
http://mingle2.com/user/enable_im
(Or >>My Account>>Settings>>Turn ON IM)

Then you would go to the profile of the person
with whom you wish to chat.

Under their picture there is an orange button
'Instant Message' which you would click on and
the request to Instant Message is sent.

If there is no orange button labeled 'Instant
Message' it could mean one of two things.

The person does NOT have 'Instant Messaging'
turned ON.
(In which case you could email them and request
they turn it ON.)

Or, the person is in 'Stealth Mode.'
http://mingle2.com/user/toggle_stealth_mode

NOTE: The mobile site has NO Instant Messenger.
~~~
If the 'Online Now' button is there, it means exactly
that, they are Online at the time, and an IM can be
delivered if all the above criteria are met.