Database Manager: fields adding / deleting

I am new to WixCode and also to Wix in general. Following the guidelines the deleting and adding fields option should be available. At the same time there is a note saying “This feature is not yet available to all users.”. Maybe this is my case. Anyway I need to modify the table/collection. What shoud I do to be allowed to delete or add fields to an existing collection / table?
Thanks,
Armando