If you're using JIRA 6.2 and above, the plugin is not compatible hence not available yet. If you're using JIRA 5.0 to 6.1.7, you can search for it from Find new add-ons and, if you click on the plugin section to expand it, you will see an announcement like this:
Seems that Zendesk has been improved in a way that you don't have to install the plugin anymore.
It hasn't been removed, it is available here
We simply "archived" it as we are preparing our newest plugin for On Premise installations.
If you are using JIRA 6.2 and above we are working on the compatibility of our add on right now, please contact us at email@example.com you need more info we'll be happy to take you through it.
If you are using JIRA On Demand you can install the newest version of our integration right away by clicking here, it's a brand new integration, completely rebuilt from the ground up!
Atlassian is working on compatibility of their framework with JIRA On Premise for mid 2015... until they release this our new add-on won't work on JIRA On Premise. The code is already ready to go, we're just waiting for the framework.
In the meantime you can use our old version for On Premise: https://marketplace.atlassian.com/archive/net.customware.plugins.connector.zendesk.zendesk-connector-plugin
As for On Demand customers it is already available in the Marketplace under the name "Zendesk for JIRA"
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