First of all I have nearly the same code in one of my programms looking for users in a group. But before accessing them I have to cast to DirectoryEntry.
DirectoryEntry deUser = new DirectoryEntry(ADPath + "/" + pcoll["member"][l].ToString())
Also please speak with your local Active Directory administrator if you are allowed to read the field by code. A colleage of mine just walked inside and told me about a field that he was not allowed to read from.