Are you talking about logging into OpenSimWorld? Or are you talking about logging into OpenSim itself in e.g. the Firestorm Viewer?
If the latter, I guess there's an explanation due. This is something that newbies occasionally get wrong.
OpenSimWorld is not the official OpenSim website. Your OpenSimWorld login is not a login for OpenSim. You can't use your OpenSimWorld login for OpenSim itself.
In fact, OpenSim is not one centralised virtual world like e.g. Second Life or VRchat either. There is no single central login for OpenSim.
OpenSimWorld is over 4,000 single, independently owned worlds, so-called "grids". It's basically over 4,000 big and small Second Lifes.
If you want to use it, you have to find a grid to join. There you have to register an account. This gives you an avatar in that grid and a login for that grid, a login for OpenSim, so-to-speak.
One well-known grid is OSgrid, one of the biggest and busiest grids and the oldest grid around:
https://osgrid.org/
There you go to "Account" > "Register":
https://www.osgrid.org/register.php
Important to know: Avatar names in OpenSim, and therefore also user names, always consist of two words. A forename and a surname. The "Username (first last)" field expects two words with a blank space between them. "Chaos Storm" will work, it's exactly two words (if there's no avatar with that name in OSgrid yet). "ChaosStorm" won't work, it's only one word.
This will give you a login for OpenSim itself, namely OSgrid.
Once you have your confirmation e-mail, open your viewer (I assume it's the current Firestorm). Now you have to enter OSgrid into it because all the current Firestorm knows is Second Life. Go to "Viewer" (menu bar, top left) > "Settings" > "OpenSim".
There's a text field where you can (and have to) enter a new grid. Enter this:
http://login.osgrid.org and click Apply.
This should add OSgrid to the list of grids and also automatically fill in most of the white text fields in the bottom half of the window.
Click "OK" which closes the window.
Now, at the bottom of the window, enter your avatar name (the one with two words) into the field to the left of the dustbin icon. Check "Save user name". Enter your password into the field to the right. Check "Save password".
Below is a box where you have to select a grid. Open it and choose OSgrid.
Now you should be able to log in.