The i-model plug-in for Revit can be downloaded in following ways:
1) Install the i-model plug-in for Revit form the iWare Apps.
2) For users of MicroStation (if you DON'T have Bentley Architecture), you can install the i-model plug-in for Revit from the Bentley SELECT download site. In the Bentley SELECT site, select "All" as the product name and "SELECT Released Products" for the download category to see the entry for the i-model plug-in for Revit.
For more instructions on installing the i-model plug-in for Revit: RevitPluginInstall
Once the i-model plug-in for Revit is installed (either by installing Bentley Architecture, or by installing it with MicroStation), you can follow this link to use the plugin: RevitPluginUse