I found my friends online here, but doesn't find any option available here for chatting with them

Under your photo, third orange box to the right, is "Instant Message". It should work unless they have disabled that feature somehow.

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