Referencing an "Addon" before use

In 
http://www.iolanguage.com/scm/io/docs/IoGuide.html#Primitives-Networking

it says:

[quote]
Note that you'll need to first reference the associated addon in
order to cause it to load before using its objects. In these
examples, you'll have to reference "Socket" to get the Socket addon
to load first.[/quote]

how do you perform this "referencing"? I'm trying to work through
the examples, and so far, No Joy!