I'm going to get in trouble here. I'm concerned that there will be resentment on the part of people I should not offend.
But fairness and future support requires thatI have my say. If I have to pay a price for being outspoken, so be it. It's in a valid cause.
The above answer that begins with the well intentioned, "For the sake of simplicity, go into your administration panel" is anything but. It is not written the way an efficient, effective and useful instruction should be written. It suffers from a problem all too common on "support boards."
Since I happen to know what that message is trying to say, I will offer it hear in the sincere hope that it influences how other answers are written.
Open your Administrator Control Panel and follow the following sequence of > (means go to or select)
> Extensions > Module Manager > Login Form (if a red button with a white x is to the right, click on that and cause it to show the greenish swoop icon for "enabled"

then click > on the words Login Form
When "Module - edit" opens, you should be in the control for "mod_login."
Scroll down to Menu Assignments.
To the right of the label Menus, click on the round dot to the left of the word "All."
Finally, scroll back up and on the right side, click on the icon for "Save."
The Log In form will appear on all pages of your web site.
Yes, those directions take more effort to write. But they have the virtue of being clear, explicit and mistake-proof. They use correct terminology, not someone's memory of what the labels and commands might be.
Those are "instructions."
Now if I have not gotten myself banned around here for talking truth to power, I'm going to go post a question of my own in what I hope will be the correct venue, and hope also that it gets a response from someone who understands what my issue here is about.