You'd have to manually add the variable in, since custom field #1 isn't in by default.
What they mean is that, in your wordlet, you could added in.
"My custom field #1 is %%CUSTOMFIELD1%%."
Well, probably not that exact wording.
Then in ubb_registration.cgi.
find:
$message = "$vars_wordlets_email{reg_notify_admin}";
$html_body = qq!<font face="$vars_style{FontFace}" size="$vars_style{TextSize}">! . "$vars_wordlets_email{reg_notify_admin_html}" . qq!</font>!;
Add below:
$message =~ s/%%CUSTOMFIELD1%%/$in{customfield1}/g;
$html_body =~ s/%%CUSTOMFIELD1%%/$in{customfield1}/g;
Untested, but should work.