At the botton of the import panel you should have:
Data format and other notes
Consider to break up your input list if you get errors, blank pages or partially imported lists: it can be a time/resource limit of your provider. It’s safe to import the same list a second time, no duplications will occur.
Import list format is:
email 1[separator]first name 1[separator]last name 1[separator]gender 1[new line]
email 2[separator]first name 2[separator]last name 2[separator]gender 2[new line]
where [separator] must be selected from the available ones. Empty lines and lines starting with “#” will be skipped. There is no separator escaping mechanism, so be sure that field values do not contain the selected separator. The only required field is the email all other fields are options. Gender must be “m” or “f”.