In the future, can you please use code tags when you post? It makes it a lot easier to read. You can find some information here:
http://bytes.com/misc.php?do=bbcode
Anyway, so the btLogin_Click method is in the user form, right? It looks like you're trying to access the main form's menuStrip from the child form... you shouldn't even be able to see that because it's private.
I realize this might be something to do with MDI (which I don't have any experience with), but I'll still try to help you out if I can.
Basically, how are you accessing menuStrip from the user form when it actually belongs to the main form? Is there some way to access the user form's parent so you can get a hold of the parent's menu strip?
That said, a better approach might be to have the user form return a login status and let the parent form show or hide it's own menu items. It would better avoid confusion like this.