What does "about now" mean?

Solution 1:

About suggests approximation, so at approximately this time. Most often used as an aside to the reader.

About now you might be wondering why you should invest in […]"

Solution 2:

The text comes from George of the Jungle

http://www.imdb.com/title/tt0119190/quotes

[Translating with a Swahili phrasebook]

Lyle: Pardon me, girls. I know you're feeling pretty hey sailor up here about now. But if you would just let me order a bowl of fried clams we can all have smallpox tomorrow morning.

So it is a tourist using a poor guidebook that renders a possibly innocent request into something rude as in the famous Dirty Hungarian Phrasebook's request for matches become do you want to go to my place -- bouncy-bouncy?

My take on this sentence is:

• Feeling pretty something : quite; very: The wind blew pretty hard.
• About now - I feel pretty tired about now = I feel quite tired at this time
• "Hey sailor, up here" - prostitutes calling from a balcony to sailors down below.

Result:

Pardon me girls, I know by now you want me to have paid sex with you, but I am very hungry so I will eat now and have unprotected sex with you tomorrow which will spread, among us all, a horrible disease brought to indigenous populations by sailors

Solution 3:

What does about now mean, in the sentence?

"About now" means "at this time."

Is it an informal way of saying?

Yes, this is informal speech or writing. I think this kind of writing style is common in email messages when the writer is guessing the mood of the recipient.