Problems with adding FRIENDS

hello, I'm new on listal and when I'm trying to check if my friends on yahoo.mail are already on Listal I receive the message "You have to delete postinstall.php before using OpenInviter".
What does this mean?
How do I delete the postinstall.php?

Thank you for your help.

Should be working now, try again and it should work.

Thank you for your answer.
No, I'm sorry but it still doesn't work. Now when I try to access the item Add friends, the page that it's opening is blank.