Download the Add-In here: https://www.xometry.com/cad-ad
Prior to installing the add-in, make sure you close Inventor.
To install the add-in, simply double-click on the ".msi" file and follow the steps in the installation wizard.
Once the installation is complete, open the Add-In inside Inventor, on the ribbon, by clicking Tools tab > Options panel > Add-Ins. In the Add-In Manager dialog box, double-click "Xometry Instant Quoting" in the list of add-ins to switch to Startup/Loaded. Click OK.