In the Installation guide there is a step where you have to unarchive the plugin. But it could happen that the following error will come up to you at that moment:
dimitar@Dimitar:~/redmine-4.1.1-7/apps/redmine/htdocs/plugins$ unzip name_of_the_plugin.zip
Archive: name_of_the_plugin.zip
End-of-central-directory signature not found. Either this file is not
a zipfile, or it constitutes one disk of a multi-part archive. In the
latter case the central directory and zipfile comment will be found on
the last disk(s) of this archive.
unzip: cannot find zipfile directory in one of name_of_the_plugin.zip or
name_of_the_plugin.zip.zip, and cannot find name_of_the_plugin.zip.ZIP, period.
This complex problem means that actually your zipfile is corrupted most likely because of some problem during the downloading. To fix it, please just delete the corrupted zipfile download it again from your license manager.