The miRBase database is a friendly website. You should only click on browse and see miRNAs which you want. After that, clicking on Magnoliophyta, you'd be able to download miRNAs of eudicotyledons and monocotyledons plants.
Thank you. I have tried as you said before. But I need to download the sequence of all reported mirs/miRs for plants to date. I am wondering if there is a way to get all of them together from miRBase, not one by one as it takes long time!