Apparently you have an older version of QC than 11 since there is TestSetFolder_Fields after this version. Might already be in 10.
But to answer your question you can get the folder object from Test Lab module's script which looks something like below. The parameter value you get is either TestSetFolder or SysTreeNode type so you can check how to use it's properties and methods from the OTA API documentation.
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Sub TestLab_MoveToFolder(Folder)
On Error Resume Next
On Error GoTo 0
End Sub