tf : The term \'tf\' is not recognized as the name of a cmdlet, function, script file, or operable program. Check the spelling of the name, or if a path
You need to include tf.exe full path. For example, you want to check in files with tf checkin command in PowerShell:
$filePath = "C:\Users\username\Source\Workspaces\teamproject\solutionname\projectname\Class1.cs"
Set-Alias tfs "C:\Program Files (x86)\Microsoft Visual Studio 12.0\Common7\IDE\tf.exe"
tfs checkin $filePath /noprompt