Всё это очень интересно, но
1) SHGetIDListFromObject НЕ существует.
2) Если вы хотите выбрать (preselect) определённую папку, а НЕ сделать её root для диалога, тогда нужен callback
3) Если вы можете удовлетвориться тем, что нужная вам папка будет root of the namespace, тогда:
FAQ: How to convert a file system path to a pidl
Aside from what SHGetSpecialFolderLocation returns in the way of pre-defined special shell folders (including virtual folders), there are a two ways that I'm aware of to covert any file system path to a pidl, one the hard and proper way, the other the easy and undocumented way. First the hard and proper way...

Encapsulate and the shell's IShellFolder interface and call its ParseDisplayName member function. This entails defining the IShellFolder member functions in ODL or IDL (Object/Interface Description Language, see the SDK for more info) and compiling a type library. But you're in luck, the EnumDeskVB example on my site contains a file named ISHF_Ex.tlb in which I've wrapped the IShellFolder interface into a type library. See the GetPIDLFromPath function in the EnumDeskVB project's IShellFolder.bas file to see how ParseDisplayName is called.

For the easy and undocumented way, Shell32.dll contains a rather neat little undocumented function that essentially duplicates what GetPIDLFromPath does (calls ParseDisplayName with the path passed to it converted to Unicode and returns the path's pidl) called SHSimpleIDListFromPath. Here's the declare:

Declare Function SHSimpleIDListFromPath Lib "shell32" Alias "#162" _
(ByVal szPath As String) As Long
Some things to consider about this function: First, as far as I know, the function is exported at the specified ordinal in all current versions of Shell32.dll (including Win95 and WinNT's v4.00.*, IE3's v4.70, IE4 and WinNT 5.0's v4.71, and Win98's v4.72), but there is nothing that says it will be at this ordinal or will be available at all in any future versions of the library. Secondly, szPath must be specified as an ANSII string in when this function is called from Win95, and as a Unicode string when called on WinNT. Finally the function's name is misleading. The function returns an absolute pidl relative to the desktop folder (as opposed to a "simple" pidl relative to its parent IShellFolder).