As you told us, you have Joomla! 3.0.3. We tell you that you need 3.2.1 or later. I think you have already told us what the problem is: you are using an old version of Joomla! which doesn't support paid extension updates: that feature was only added in Joomla! 3.2.1 which was released a year after 3.0.3 that you are using. So, no, you are not using the latest Joomla! version. You are using a very old one, which has known security issues. Please upgrade to Joomla! 3.3.3.
If Joomla! Update can't find the update, please install our free Akeeba CMS Update component. I have written both the core Joomla! Update component and Akeeba CMS Update. Think of them as version 2 and version 3 of the Joomla! updater. Once I have enough users having used Akeeba CMS Update successfully I am going to contribute it to Joomla! (if they want to take me up on the offer, that is).
Nicholas K. Dionysopoulos
Lead Developer and Director
🇬🇷Greek: native 🇬🇧English: excellent 🇫🇷French: basic • 🕐 My time zone is Europe / Athens
Please keep in mind my timezone and cultural differences when reading my replies. Thank you!