It is simple:
Comment on any post saying you are interested and include, (1) a name/username you wish to be known as, (2) your URL of your public blog, (3) the name of your chosen charity and (4) your charity's donation page URL (must be legit!).
That is it.
Blog-o-rama 2011 has raised:
World Time Clock
Here is a link in case you are not sure what time Blog-o-rama 2011 starts in your time zone.