If you have played with the early releases of PowerShellGet and the PowerShell Gallery (www.powershellgallery.com), you may still have a module called "PSGet" on your system. If so, this module may conflict with the final version of PowerShellGet because it provides the same cmdlets but with different parameters and functionality.
Here is a line that helps you find out whether the PSGet module is present:
PS> Get-Module psget -ListAvailable
If the command returns nothing, then you are safe. Else, you should remove the PSGet module.