For being eligible for dagpenge, you need to have been member in A-kasse for at least 1 year. And also you must meet the income-requirement. The requirements can be found on this page.
If you had paid to unemployment insurance in Portugal you could probably had transfered your earned rights to the danish A-kasse. However as you can see in the link, there are some requirements for being able to transfer rights, and it seems that you will not be able to meet these requirements.
So, because you can not transfer rights from the Portuguese Unemployment system to the danish A-kasse, then you need to start from "scratch" in Denmark. This means that you must be member for 1 year, and have normal income for at least 1 year, so you will be able to meet the income-requirement.
Anders Weber, www.a-kasser.dk