To be able to add Items to a Tender you first need to add Items to the Procurement module:
Create Item. Add link Create occurrence of an Item by adding it Items
https://drofus.atlassian.net/wiki/pages/createpage.action?spaceKey=LearndRofus&title=Add%20or%20Move%20Items%20to%20an%20Tender%20Group%3A Create Tender Groups