After you have applied for the ladder, created a team, joined an event, and received a challenge; you must accept the challenge before the match can be played or any results can be submitted. Navigate to the matches section where all challenges and pending matches are displayed.

On that page you will see an option to confirm the challenge, options for actually accepting or declining the challenge are on the following page.

Although some changes are planned for this form, it should appear something like this, simply click accept challenge to confirm the match. The match will then be displayed in pending matches on the event homepage, until results for the match are entered.

You can also decline the challenge on that same page, just keep in mind there may be a penalty for declining which will vary by event.