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When you create a custom block, the default behaviour of XOOPS is i) to allow registered users and webmasters to see the new block and ii) to NOT allow anonymous users to see it.
You need to go to admin => groups => anonymous and set the permissions so that anonymous users can see the custom block too (if that's what you want). You may also need to set what modules the block is actually visible in.
I suggest you download the Operations Guide, which you can find
here. Have a read of the sections on groups and blocks.
With the start page, in admin => preferences you can change the start page to be any module that you have installed. I suggest you start with something easy like 'news' module, but if you want to have a custom HTML start page then install a html content module like tinyD.