if you want to attach something to an existing swiki page,
just go to that page, for example Code Pool SC2

hit the [Attachments] link
click the button that says open... and select the file you want
to upload. Then hit 'upload'.

go back to the page you'll see there is a link to your file.
if it is a .txt or .html it will be readable directly, otherwise
a click on the link will ask if you want to download that file.

if you want to update a file that is already there, select
'do not add a reference', so that you do not get a duplicate reference.
all past versions are kept, so everything is safe.

to create new pages, etc., see how to contribute or easy start
also consider to read the copyleft information

and remember that several clients might have problems with blanks and special characters in filenames, so better use underscores instead.
Also remember that uploading copyrighted-mpeg files is a violation of the permission to use the resources in a cooperative way.