Here's how you would do it:
1. In the Visual Basic Editor, go to the menu bar and select View-->Object Browser.
2. Click the drop-down list that contains the words "All Libraries" and select "VBAProject". This will display your modules in the pane on the left.
3. Click the module or class that contains your User Defined Function.
4. Right-click the name of your function and select Properties.
This will open the Members Options dialog box, where you can enter your description and point to the correct Help context.