"SEO" in this article stands for Search Engine Optimization (e,g., maybe you don't want your warning against some site or person to improve their search engine ranking positions on Google, yahoo, Bing, etc.).  Won't prevent search engine bots from indexing your post with that text, but clicking the link won't aid SEO.

The nofollow tag is basically a notice sign for search engines saying “don’t count this.” Per article: "A no follow link is a link that does not count as a point in the page’s favor, does not boost PageRank, and doesn’t help a page’s placement in the SERPs. No follow links get no love. Theirs is a sad and lonely life.". By removing referring site as well, makes it harder to trace back to your own sites.

On booklikes, to insert the nofollow tag, create your link* then edit the HTML (the last edit option on menu line with block quotes, bold, italics,merc. that looks like a less-than-sign greater-than sign) and manually enter it.  Add the rel="nofollow" right before the link's closing > .

*Normally, just typing in the link starting with "http://" creates a URL link in a booklikes post.  But, if your created link appears in your post as, for example, "http://blog.booklikes.com" instead of http://booklikes.com -- try just adding a space immediately after as a quick fix.  Signifies to booklikes the url is finished ( and allows more complicated links with = ?  / , etc.).  If that doesn't work, highlight link text and click the chainlink icon to insert relevant info (if still nothing working, resort to the HTML editor and manually type in the code).

To set the reading challenges you're likely seeing post to your dashboard from your followed booklikers, it's at http://booklikes.com/goodies/readingchallenge .  If you lose this post/link, it's under "goodies" on the main menu (so is the Reading Challenge widget code for your blog, direct link http://booklikes.com/goodies/widget ).

Click the "read more" if you're in dashboard view and you'd like creenshots (you'll still stay on booklikes without being redirected to another website/blog):

The reading list feature fo booklikes is at http://booklikes.com/apps/reading-lists or you get to it by selecting "apps" from the main menu.   Screenshot:

If you tell booklikes to add all books from the reading list, it creates a shelf named the same and adds them.   Unlike listopias on goodreads, the lists don't show on the book pages and the list creators manage (you can comment to request updates or discuss something).  No voting campaigns or paid promoters gaming the lists, yay!  Example screenshot from reading list Read Alike List for "How It Went Down" 2015 Coretta Scott King Author Honor Book :

How do I tell from the book page, when editing my shelves or search results that I've already shelved a different edition of the book?

UPDATED WITH AN ANSWER:  ( thanks to Tannat's comment) On search results you'll see a vertical green bar if you've shelved an edition.  (Still not sure about on book page).  Screenshot showing bar:

For example, when I am updating my shelves, someone's post has me click open a book page or maybe do a search on ISBN of book in hand.

(When my edition has been correctly combined, of course.  I realize if there are stray editions that I first need to click "report" and ask a librarian to combine with correct work.  And that there's a known bug where the narrator name as author causes audiobooks to not combine with their print/ebook editions -- again that should be reported to get combined and edition edited so narrator says narrator instead of author.)

The large bookcover at left (dashboard view) you get by choosing "post" on book pages, "update and publish" on currently reading display or by pressing the large "plus" sign when creating/editing posts. 

If in the text of your posts you want to link to book page (like Runaway Robot - Lester del Rey ) or with book cover thumbnails like:

.       

 then read on for screenshots and instructions.