In the review request email, in the first line, there is a link to the peer-review form (e.g. https://www.jmir.org/reviewer/submission/166100?key=xxxxxx) , where you have to click the green CONTINUE button under "Accept or Decline Review Request after selecting "I agree to review this" radio button.
After accepting the request, you can find the paper on the same URL under "Download Options" - click any of the download buttons (see Where do I find the papers I've been asked to review? and also How does the JMIR peer-review form look like?)
You do not need to login - simply click on that link in the email.