I have been looking through the posts on ERA and haven't been able to find anything on how to add existing characters to ERA. Do I have to create them from 0th level and level up to the current level then edit the char sheet to put the right number of ranks in each skill?
You can do it either way. For NPCs I've always just leveled them, but I believe to input the data you would just level up the necessary number of times without spending the dev points. Then edit each skill to input the right number of ranks. You can also skip stat gain rolls, and then manually adjust the temp stats.
If you are creating a new high level character the advantage of leveling up the long way is that it will ensure the character is built in keeping with the rules, and you get a detailed record of each level up stored as a part of the character.
If you are inputting already existing NPCs or PCs, then without detailed records of the characters prior stat gain results, the dev points available at each level won't match how the character was originally built. At that point working within the ERA level up mechanics is very likely to result in something different from what's on the existing character sheet. Your best option for that type of character is manual entry.
You can always create a test character in ERA and then mess around with manual input first to make sure it all works right.