how do I calculate alternative present values?

That's too ambiguous as there are many things this could be applied to. The fact that they're alternatives has little meaning in how it's done, because it just means you have to do two different things and then compare them. How each is done could be different.

They could be lump sum, or annuities, compounding in different ways. There's also charts to be used, financial calculators, algebraic equations -- which do you prefer? There could be a series of cash flows based on some other information that you have to put together first. And I don't even know which part you've having trouble with.

I quite literally mean this could be anything. Could you get a little more specific? Is this for homework?