Structure is installed like the most other plugins. With JIRA 4.3 or later, or when you have Atlassian Universal Plugin Manager installed, you can locate the plugin at the Atlassian Marketplace and install from there.
Alternatively, you can download the plugin JAR manually from Atlassian Marketplace and place it into plugins/installed-plugins subdirectory under your JIRA home, then restart JIRA.
Overview
Content Tools
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Tasks