I believe there is a set up fee you have to pay. I have a £75 UK one, and the terms and conditions say something to the effect of their being a £5 set up fee, but essentially that this has been included in the £75 amount to offset it. I.e. I think Google is saying that the voucher is really £70 - the additional £5 is included to cover their sign up free. - Its really just a work around so that Google can claim that its a completely "free" trial as such (note that the t&c may say something different in the US). Not that I'm complaining - its still a great deal. One thing I have noticed - if you ignore it they sometimes send out higher amounts (I got a £25 voucher, then a £50 one, and now a a £75 one - about $120). However it can take many months for the next voucher so I don't really think its worth waiting for.
You should contact Google Adwords sales staff if they don't get in touch with you within 3 business days. I got a call from them about 2 days after I created the account. They will go through you how to use adwords and apply the credits hopefully.
I received one as well, it's a start, with good keyword research, you can obtain a considerable amount of interest mphorton2