Structure is installed like most other plugins.
Open Plugin Manager, search for "Structure" by ALM Works on the Atlassian Marketplace and install from there.
Alternatively, you can download the plugin JAR manually from the download page and either place it into plugins/installed-plugins subdirectory under your JIRA home (then restart JIRA) or use "Upload Plugin" link in the Plugin Manager.
Congratulations! You can now spread the word and help users get started with Structure – see Getting Started with Structure.
If Structure Plugin Remains Disabled
It is possible that after you install Structure or enable it from the Plugin Manager, the plugin remains disabled. An error may or may not be shown. If you refresh Plugin Manager page within 5-10 seconds and Structure is disabled, you've got this problem.
See Structure plugin won't start article for possible causes and solutions.