When you add it to the main page (you probably mean 'site root'), you get an url in the browser
( something like /++add++mytype)
Do the following:
1) Copy the url
2) Add a folder, then copy the part you got in 1)
3) This will give you an URL like: http::/mysite/myfolder/++add++mytype
4) Press enter.
If you can add the content type now, 'the action' is not properly declared.
If you still can not add the content type: Maybe the user does not have the right, ... try as 'administrator' if you did not already....