The data structure in ADvendio is quite complex as described in 3.1 Overview of Data Structure. Please make sure you are familiar with it before using any of the inventory tools. To find out what you can do with the ad spec and rates manager watch this short Ad Specs & Rate Manager video.
To manage your inventory we offer you multiple tools. The AdSpec and Rates Manager is just one of them. Other wizards you find in the inventory section of the Overview tab.