center the output of a list

This is a fairly intricate aspect of HTML, so I can offer some simple suggestions, but ultimately you will probably need to familiarize yourself with CSS, which is a set of instructions on how to format the display of HTML.

I suggest you read this article that simply explains how CSS works with WordPress: Simple CSS Techniques for WordPress

First, the “center” tag isn’t used any more, and it doesn’t work very well anyway. What you need to do to affect the layout is to add CSS rules to make your changes. This can be a bit complicated. The “default” template displays the Participants Database records in a “table” that boxes everything up in a grid. You can easily center the contents of each grid (table cell) with something like:

.pdb-list td?{
text-align: center;
}

Hello,

thank you, that works fine. But I dont need the lines of the table centered, but the whole table which is aligned right and that looks not good.

I tried with:

.table, table {
text-align: center;
}

and

table {
text-align: center;
}

but had no success.

Well, I hate to have to answer like this, but I can’t give you the CSS code that is guaranteed to work, so much depends on the HTML and CSS that is on the page already. Taking some time to learn more about CSS is your best bet for understanding how to do that. You can also try doing a general web search for something like “CSS center table” and probably find several tutorials on how to do that. This isn’t specific to the plugin at all, just general knowledge about HTML layouts.