How to Top Up Skype Credit on iPhone
You can’t do it directly from your iPhone. Got the note below from Skype’s web page:
You can’t currently purchase Skype Credit directly from the Skype application on your iPhone. However, you can purchase Skype Credit using your iPhone’s internet browser, in the same way that you would from your desktop.
So, fire up your Safari on your iPhone, login to Skype and buy the credits.