Microsoft Project Side

Redmine Connector Plugin Installation
If your office(Not Windows OS) is X86, download the X86 file, else download the X64 file. Go to Download
First, your computer must be online while the installation may download dependencies(.net framework and vsto runtime) from internet.

Double click the redmine_connector_xxx.exe file to start a installation.
There is a Redmine tab in top of Microsoft Project after installation is finished if everything is OK.
Setting Redmine Host

In Redmine tab, click Setting to open the Redmine Host dialog, then type your redmine host and click confirm:

Load Redmine Issues to Microsoft Project

When the redmine host is setting correctly, click Open to open the sidebar in the right:

Type your redmine login name and password, then click login. Then chooise a project after login:

Select the columns showing in Microsoft Project current table, then click Load button.
There difference between Load and Load and Merge is:

Load: Delete all the tasks and reload all issues match the filter.
Load and Merge: Load all issues match the filter and update the tasks or append new issues to the tail.

The redmine issues are loaded as below: