If you are having trouble with a Joomla extension, the developer might ask you for your Joomla version, PHP version, and what type of server you are using (e.g. Apache, IIS, etc). Here is a quick way you can get that information.
Go to Site then System Information
Go to Site then System Information
For Joomla 1.5, you would have to go to Help then System Information.
Adminsitrator Footer
You can also quickly find your Joomla version in the Administrator Footer. For Joomla 1.5, you could look in the top-right corner of your Administrator section.