How do you include a poster in an academic resume?
In academia, one lists things related to accomplishments and dissemination of research in the CV.  This typically includes things like papers or preprints, software, as well as presentations one gives.  In particular, your list of presentations gives one piece of information into how active you are in your research community.  So yes, assuming your poster presentations are academic/research related, it's appropriate to put them in your CV, though in-class presentations are typically not included.  

General comments:  a CV is different than a resume so there's a lot of flexibility in what you include and how you format it.  Still, there are some dos and don'ts.  You can find loads of example CVs online, but it might be most helpful for you to look at sample CVs of young researchers (students, postdocs) in your field for some models.

Specific comments: there are different ways you can organize your presentations, but my suggestion is make a section titled presentations, and within that two subsections, one for oral presentations or "talks" and one for poster presentations.  Then for each presentation, list the title, venue including conference name and date.  E.g.


From Cervantes to Swift, 33rd AEIOU Conference, University of Lillipilut, Mildendo, Lilliput, April 1, 1726.