Ah, the magical question that every developer wishes they knew the perfect answer to.
Unfortunately there's not really any easy way for us average developers to make our app famous over night. Simply put, you need to get the word out as much as humanly possible, however this is far easier said than done. You can do things like submit it to review sites, though they tend to only look at rather polished and unique apps that take quite some time to develop. Really any way you can think of to direct attention to your app, but it can lead to a slippery slope. Don't spam or post your app in places it shouldn't be posted, it will do more harm than good. I wish I could give you an answer that is actually helpful, but it's just not an easy thing to do unless your good friends with a celebrity who has millions of twitter followers and can get them to tweet about your app.
Your app will have a harder time than usual getting noticed because of what it is. There are plenty of very well made calculator apps floating around the market that have a very well established user base. That's not to say it's impossible, it will just be difficult.
I gave your app a whirl myself in the middle of typing this. It does what it is meant to quite well. There are two suggestions I have to make sure you don't get bad ratings from people (Because many Android Market users are funny and will 1 star an app for one little thing wrong). The first being to add a "clear" button to erase everything, and the other being to add a check on the backspace button to make sure it doesn't try to delete a character that's not there (I'm sure you know about this since it will crash the app).
Other than that I'd say it works very well. Good luck getting more users!