Click on the search icon at top right.
If you type some words, you’ll get a list of posts that include those words, even as substrings of URLs. But if you type a number that appears as an element of a URL, the post will not be found. Which is very unfortunate, because numbers in URLs are a way to identify specific Memrise courses!
For example, I’ve made a few posts that link to my course http://www.memrise.com/course/750078/tropical-coral-reef-fish-identification/
You can find those by searching for “tropical coral reef” - and it will highlight the URL because those words appear in the URL and match the search. But if you type “750078” in the search box, you get no hits.
A couple of posts I’ve seen have comments that link to my Hebrew course, http://www.memrise.com/course/702355/hebrew-reading-and-language-intro/8/ - but of course if I just search for “hebrew” or “hebrew reading” I get all sorts of hits on other posts that don’t link to my course. It would be really useful if I could search for “702355” and find only the posts where there is a link to my course, but that doesn’t work.