how to display a string in admin_notices hook from a class plugin

Take a look at what happens when you activate a plugin. That is, watch the event with something like HttpFox or Wireshark. After activation the page is redirected back to itself with $_GET parameters appended. For example:

wp-admin/plugins.php?activate=true&plugin_status=all&paged=1&s=

Notice, activate=true?. Your code isn’t working, I am pretty sure, because there are two requests involved. You will need to compensate for that redirect with, for example, a transient or cookie. You may be able to add your own $_GET parameter but I do not have the time to research and cook up any of that.

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