MarketPress, GoogleCheck Out Not Working. Please help.

To whom this may concern:

A few days ago I purchased your marketpress plugin ($39 for month, as a trial). I was initially impressed with how easy it was to set up the store, but knew there would be a potential problem, and there is, when setting up the payment option. I chose to go with google checkout. I have another online store I had someone set up for me, before I discovered wordpress. On that site (http://skin-lighteningstore.com), I have google checkout, I have a goole merchant account set up for that.

So what I did is this: I designed a wordpress site (http://skin-lightening-store.com) and purchased your marketpress plugin. All is working well, except the google checkout. Please answer the following:

1). Can I use the same google merchant account on several sites? I was told yes!

2). I followed the instructions that accompany your marketpress plugin, looked up the merchant id and the merchant key. I set that up. What I don’t understand is this:

What is the difference between sandbox and live? What is meant by this, as per your instructions?

You must setup your API callback URL in Google Checkout to be able to process orders.Login to the Integration page of your Merchant Center sandbox or production account. (You must set it of each)

[ What does set it of each mean?] I didn’t see that option anywhere in google merchant account.

3. This is the message I get if I try to run the a test purchase on the site with your plugin:

There was a problem setting up your purchase with Google Checkout. Please try again or select a different payment method.

Bad username and/or password for API Access.

4. What about setting up a security option for the site, so consumers know their purchases will be safe?

I have a ton of other projects to work with, and like the other plugins your company has to offer, but I’m not sure if to proceed with others given the trouble I’m having now.

Please get back to me at your earliest convenience.

Thanks so much,

Anabelle