Yes, that can be more than just a little confusing. I'm not sure I can help much,, but I will try to explain it the way I understand it.
The "userid" is the name of a "machine user", and each account can have an unlimited number of these. This is the "users" that manageers and FTP/Shell account on the server, and under whose permissions CGI programs are run by the webserver (suEXEC for the technically minded). Depending upon when you signed up for your account, it may, or may not, be the same as your "WebID" (which is generally what you are referring to as the "panel userid) - more on that later!). Older accounts, created back when you could select your own "WebID", often have the same "user id" as the "WebId" (if you set it up that way).
The "panel id" you are likely referring to is actually either the "WebID" assigned to you when you created your account, or the WebID of another DreamHost user to whom you may have assigned account privileges in the control panel (to help manage aspects of your account). Newer accounts have a "WebId" created for you by the system, and it usually is a combination of your first and last name's first three letters. DreamHost is actually pretty much phasing out the use of the WebID in favor of the email address, and at present you can sign into the Control Panel using either the WebID or your "primary" email address associated with your account.
Neither of these should be confused with the "user is" that manages an FTP or Shell account though, as I mentioned before, there are cases where they may be the same (older accounts).
All that said, you use either the "WebID" or your the email address associated with your account to log into the Control Panel, and your "User ID (s)" to log into your FTP or Shell accounts.
If you go to that address, and look further down the page, you will see two sections - one for "Contact Info" and one for "Security" From the "Security" link, you can change the password associated with your WebID, but your WebID cannot be changed. You could, of course, create another, but there is no need to do that.
The easiest way to discern exactly what your Current WebID is, is to review your "Welcome" emails from Dreamhost (it's in there), or to write support should you not be able to find it. Note that you really don't need your "WebID" for any function anymore, as you can always use your associated email address in it's place (which has been the case for quite some time now).
There is additional information about much of this in the DreamHost wiki:
As for getting at your "Stats", the easiest way to sort your password problems there, is to just go in the Control Panel to the Site Statistics page, and click on the little "wrench" icon under the "# of users with Access" column. This will take you to a page where you can change the password for any existing "stats" users, and/or add additional user/pass combinations for accessing the "stats" directory of your site (often useful to allow a client to see the stats without allowing them any other account privileges).
I hope some of this has helped make things more clear, though I recognize I might have only succeeded in confusing things further!