The Marketplace is not available as you cannot install add-ons with an OnDemand instance at this time, you can only use the existing ones that are bundled with the system. For full details you can check out the OnDemand Plugin Policy.
Yes, all plugins on marketplace are available during trial. Assuming you are on 5.X version, the marketplace can be accessed via Plugins -> Find new plugins.
If you still do not find the link, then open a new browser and go to market place and download the plugin you need and upload the plugin manually to jira.
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