There are two answers for two very different scenarios.
Tell them straight. They are paid for getting people into the job, and just want to know about your situation with regards to availability. They will usually push for the best fit salary for your skillset because this keeps them sweet with both you and the prospective employer.
This is much more subjective as some employers will snap you up if you've been made redundant due to the loyalty factor (you're not quitting), whereas some will use it to their advantage. In reality, you probably wouldn't want to be working for the latter anyway.
As a rule of thumb, only tell them if you feel like it won't impact your chances. Once you have the salary and planned start date, you should be able to tell them that you were made redundant when they ask, otherwise you can use
The company is undergoing a few changes and it's time for me to move on
As for whether to hide it entirely, it will be pretty obvious when they ask for a reference, or if an enterprising HR bod just looks up the previous employer and finds an industry news article.
Please note, this is a UK-centric view of the situation, other cultures may have different recruitment styles