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Hey,
First, you original question is rather vague and that could be put down to your lack of knowledge regarding Xoops.
I'm going to make a stab in the dark and try and answer you and I hope that I'm going in the right direction.
When you create a new custom block, the information is stored in the database and not in a flat file (I'm assuming this is how you think custom block information is stored) and to access this custom block and to display it, is done through block Administration.
In the Block admin, you have the ability to select on custom modules to be displayed within the listing. You can edit the block from there and select which position you would like the block to appear, just like another other 'module' blocks.
You can also, if you know PHP well enough, venture into the world of templates and SQL queries to get block information and display the block via smarty.... But, I don't think this is where you would like to venture..
Tall me if I have got it wrong and we can discuss this more and hopefully get your question answered.