Your question is good but if you could provide some more information it would be easier to give you a proper answer.
the conversion rate you have from download to sign up sounds a bit fishy as many here said before me. Usually a user who decides to download a software is already interested to try the product.
if you would tell us what marketing tools you used that would help.
I would also recommend you to try a different aff program and see if you can get better conversions by doing that.
I hope this helps 