How do you turn on the metronome of a Casio 3100 keyboard

You should be able to find the information in the manual. Here's a
list of Casio keyboard manuals on their website. You need Adobe
Acrobat to read the manual after you download it.

http://world.casio.com/emi/download/en/manual/

If your keyboard model is not listed, Casio also provides a technical
support 800 number for musical instruments here:

https://www.casio.com/support/contactus/phonenumbers/

The manual says you can turn on the metronome but not how to do it

You're not alone. I found two other posts of similar questions on
Google Group boards, but no answers. I'd say call the 800 number.
Although I tend to be a little cynical about customer service by
telephone, I think it's worth a try.

Another option is to find or ask someone to recommend a local music
store that carries Casios or a local musical instrument repair shop.
If no one here has an answer, call the store and/or shop and see if
one of the sales people can help you out.

I also found this 2-page PDF document on a Casio site; it includes
instructions on operating a metronome on a Casio keyboard. However,
there's no info in the file itself to indicate what model is being
referenced. With the slight chance that these directions might be
helpful in some way, I'm adding the link to the file:

ftp.casio.co.jp/pub/world_manual/emi/en/lk_50/lk50_e_07.pdf