If you're trying to work with external functions in DLL and want to make sure you have the name and case of the function name correct, InspectExe is quite useful for viewing all of the exports from an EXE, DLL or OCX. The utility adds several panes onto the properties dialog in Windows Explorer.
If you're having trouble with a DLL, it may be because it has depenencies on other DDLs, and it's actually another DLL that is actually causing the problem. A real useful utility for resolving that is Dependency Walker. It adds a menu item to the right mouse popup menu in Windows Explorer that allows you to launch the program to process the selected DLL.