Werewolf for JIRA is a common JIRA plugin. So it could be installed like the other plugin through Universal Plugin Manager.
To install the plugin you need to logon to JIRA as an administrator, and in the Administration\ Add-ons\ Manage Add-ons find our plugin by name: Teamlead Werewolf for JIRA and install it.
After installing the plugin through UPM go to menu Add-ons\ Werewolf and specify a group of users that will be able to work by other user's profile.
Then you can choose user by clicking Menu/ Werewolf and see this user's system without entering Login neither Password.