To report a game in a cup, Simply enter that cup's page.
If you want to report a game in a group, The 'Report a Game' link will appear under the group you are in:
(https://www.tus-wa.com/images/guides/reportCup2.jpg)
If you are in the knockout stages, The report link will appear under your box there:
(https://www.tus-wa.com/images/guides/reportCup.jpg)