You don't need no software to write a business plan. Grab a book. There are even business plan samples and templates out there, just spend half an hour on Google.
I agree with Oleg, just write one on your own. There are a lot of templates out there and you can adapt them for your needs. Do your financials in a spreadsheet and embed them into the word processing document to complete the plan.
One of the benefits of writing a business plan is that it gets you thinking about all facets of your business. Business planning software, by its very design, dumbs down the process so you don't have to think things through as much.
Who are you writing your business plan for? Your own team or investors? In either case you might want to avoid getting bogged down in a long paper document and instead focus putting together a set of slides that convey the key ideas. I like the recommendation of Sequoia Capital in which they recommend 15-20 slides: