Right click on the name tab of the worksheet that contains the button, choose "View Code". In the VB editor that comes up you will see two drop-down menus, one will say "(General)" and the other will say ")Declarations"). Click on the arrow for the "(General)" box, and you should see the name of your button in that list. Select that button. You will now see some code in the window that says something like:
<pre>
Private Sub CommandButton1_Click()
End Sub</pre>
Put your code to show the userform in here. e.g. if your userform is called "UserForm1" then you put this line of code in to show the userform:
<pre>
UserForm1.Show</pre>
Go back to your work book and hopefully if you click your button the userform will show. If nothing happens that check the toolbar called "Control Toolbox" and make sure that the "Design Mode" button hasn't been pressed.
HTH