Thanks for Help_everyone and Marcd for the replies!
It seems to me that the import feature was having troubles by the time I´ve written my original message.
I had tried with the Firefox extension and today I tried with the Chorme extension, after reading Marcd post.I finally got to a page where I could upload a csv file generated by LastPass, wich I couldn´t reach when I tried with Firefox. I´ve successfully uploaded it and imported everything.
But then I decided to try with Firefox again, and this time I was able to reach the page where the upload should be done. Didn´t really upload the file again, but I think that the process would have been comlpeted successfully with Firefox too.