If you are trying to install a module that is listed on the central repository for PS content called PowerShell Gallery, you need to install PowerShellGet. Then the command will be available. I'm currently using PS 4.0. Installing PowerShellGet did the trick for me.
Source:
With the latest PowerShellGet module, you can:
- Search through items in the Gallery with Find-Module and Find-Script
- Save items to your system from the Gallery with Save-Module and Save-Script
- Install items from the Gallery with Install-Module and Install-Script
- Upload items to the Gallery with Publish-Module and Publish-Script
- Add your own custom repository with Register-PSRepository
Another great article to get started with PS Gallery.