What I was saying you could do to test that the url is correct, is create an index.html file, it can say anything on it. Upload it to the same place as your index.php and index.html will be the default homepage when you enter your url in your browser. Do this to see if you can see the index.html page that you uploaded.
If you can see it, then your problem is XOOPS related. Did you create a database for XOOPS before trying to install?
You will still have a tar file on your system because it's created when you do the initial unzipping. But you should have another folder or two there also with files. The folder you untarred into should be filled with the untarred files and the original tar file. When I untarred that file myself on my home computer, I found extra files with it. Look for a readme or install file.
To add another banner where it says little slogan, you would need to add the img code into your theme.html file. You need to look at that file and find where that spot is in it, should be showing above the logo image code and add in the code for the banner/image there. If you need help with knowing where to put it, just post a copy of your theme.html page here and someone could point out where it will go for you.
The error comes from your webhost, max100. You could try putting an index.html in your server root directory to make sure but it should come up saying the same thing. That means you are either using the wrong url, or your webhost provider hasn't set up that url for you to use yet.
Do you have permission to use the files in that folder that you installed XOOPS to? It seems to be a permission problem by the error and that would be for your access as a user to all the folders and files in the directory you are using for xoops.
I tested this package and it extracts correctly. Just make sure you extract it to a folder that you create for it, first. http://www.active-link.com/software/