I was testing and playing around fiverr.com. I created my offer and then I was stuck to find option when I wanted to create extra gigs into my offer just like other people selling the way.
I was feeling bad and there was no easy way around inside user's panel about it.
Reasons:
You can add extra gigs to your newly created gig after 10 sells are done with it. Why? Because you need to reach level 1, 10 sales withpositive feedback in a month required to reach it.
More Information: http://fiverr.com/Levels
Hopefully, if you are new there and stuck then it should solve your problem now :)