I have a code for 3 months of amazon music unlimited. where do I enter it?

hi Shelagh.
this is all i could find.
At $9.99 per month, Kindle Unlimited costs about $120 per year. E-books on Amazon can vary wildly in price, from $0.99 to hundreds of dollars.18 Jul 2014

Amazon Kindle Unlimited: Is It Worth It ' Time

time.com/3006110/amazon-kindle-unlimited-value/
or read up more at this link.
How much per month is kindle unlimited subscription Google Search

Apparently not available in kindle unlimited, as the description does not show the Kindle unlimited logo or banner on the Amazon web page shown below and is for sale for $39.99:

https://www.amazon.com/Chronological-Gospels-Seventy-Ministry-Messiah-ebook/dp/B00F6C5VEC/ref=sr_1_1_twi_kin_2?s=books&ie=UTF8&qid=1509312775&sr=1-1&keywords=chronological+gospels+michael+rood

There are four other ways to search for Kindle unlimited books as described below:

https://ebookfriendly.com/find-kindle-unlimited-ebooks/

see if this link helps you.
https://www.google.co.uk/webhp?sourceid=chrome-instant&ion=1&espv=2&ie=UTF-8#q=How+do+I+get+a+6+month+Amazon+Kindle+Unlimited+subscription

At $9.99 per month, Kindle Unlimited costs about $120 per year. E-books on Amazon can vary wildly in price, from $0.99 to hundreds of dollars. During 2013, e-books on the Digital Book World best-sellers' list mostly sold for between $7 and $8 on average (the price in the most recent recorded week in 2014 was $7.52).18 Jul 2014

Amazon's Kindle Unlimited Is Worth It If You Read This Many Books ...

Kindel unlimited cost Google Search

They are different programs. Prime has a loaner program for books. Check online for unlimited pricing.

At $9.99 per month, Kindle Unlimited costs about $120 per year. E-books on Amazon can vary wildly in price, from $0.99 to hundreds of dollars. During 2013, e-books on the Digital Book World best-sellers' list mostly sold for between $7 and $8 on average (the price in the most recent recorded week in 2014 was $7.52).18 Jul 2014

Amazon's Kindle Unlimited Is Worth It If You Read This Many Books ...

https://www.amazon.com/gp/help/customer/display.html/ref=hp_search_rd_gw?__mk_en_US=%C3%85M%C3%85%C5%BD%C3%95%C3%91&help_keywords=cancel+kindle+unlimited&search=true&nodeId=201511640&kwHidden=true&sprefix=cancel+kindle+u%2Ccancel+kindle+unlimited%2C2&locale=en_US

key in "Kindle First" There are books that are new and you can get one a month free if you have Kindle Prime.