To flesh out my previous answer;



Your content  Type needs to inherit (have a parent) from a list  or Folder Content Type. Of the List Content types  you could use as a parent, the 'Item' content type  is often a good choice as it only has one column of Title, leaving you to add  your own choice of extra columns. Some of the other pre-defined ones might be useful though, so check them out.



You can put your new Content Type in any group you want (Custom Content Types perhaps?). You'll then notice that that group shows in the drop down box for adding to the list, but only custom  content types derived from a list content type (or folder) will ever be shown.



So, in summary your Content Type needs to have the right parent Content Type for you to add it to a custom list.