You cannot install Folder HelpDesk directly in the Office 365 Outlook. Instead you have to create a shared mailbox in Office 365 Exchange and then use with Folder HelpDesk.
When the shared mailbox is ready for use, each Folder HelpDesk user needs to add it to his/her Outlook. It might be added automatically, but if not, the Tips article also describes how to add the shared mailbox manually.
Install Folder HelpDesk in the shared mailbox:
Install and configure Folder HelpDesk from your desktop Outlook as described in the Folder HelpDesk manual and demonstrations. Select the Office 365 Exchange shared mailbox in your local Outlook as the Outlook installation folder.
Now all users who have access to the shared mailbox can reach and work with the helpdesk tickets.