Here is how you can make your siggy appear: -
When you want to add any reply, then you type that reply in a box. Below that box, see if you find a check box called "Show Signature". If it is there, then see if it is selected. If not, then select it. Now type your reply and click Post Reply. You will see the siggy.
It is possible that you do not see "Show Signature" check box.
In that case, go to your profile by clicking "Settings" link near top-right. You will see a message in red colour, which says that you did not activate your account. After that message, you will find "Click here" link. Click on that "Click here" link.
Log out. Go to your email, which you gave when you registered with IF. You will find a mail from IF. If you do not see the mail in Inbox, then check Junk Mails folder. Open the email. In that there will be a link. Click that.
You will find a message that your account is activated. Log in into IF. Now, if you want to post any reply, then you will see "Show Signature" check box below the "Post reply" box.