I'm sorry to hear that. I hope that your further experience with WordPress will be good. My feeling is that once you've mastered the use of Custom Menus, you should be good to go with my plugin.
The questions I asked were intended as "try this and please report back". I'm sorry that these have not solved your problems.
I have retraced the steps, and have used my plugin version 0.8 with both WordPress 3.6 and the Twenty Eleven theme. This should be your base case.
In a final attempt to help you, I've listed the steps to use the plugin with the Twenty Eleven theme below.
- Please disable all plugins except Category Posts in Custom Menu
- Create a post with title "My test post" and add a _new_ category "CPCMtest"
- Save the post
- Go to Appearances > Menus
- Click "create a new menu"
- Give the new menu the name "CPCM"
- Add the category "CPCMtest" by opening "Categories", checking that category and clicking "Add to menu"
- Open the menu item and check "Create submenu containing links to posts with this tag."
- Under "Remove original menu item" choose "Never"
- Do not change anything else about the menu item
- Scroll all the way down and check under "Theme Locations" the checkbox "Primary Menu"
- Save the menu
If you now look at your blog, you should have a menu with only one item: "CPCMtest". If you hover over that item with the mouse, it should open, and a post is displayed titled "My test post".
Now, to use Dropdown Menu Widget:
- Enable Dropdown Menu Widget
- Go to Appearance > Widgets
- Drag the Dropdown Menu Widget to FOOTER AREA ONE.
- __IMPORTANT__: The Main Sidebar in the Twenty Eleven Theme does not show on many pages! This is not my fault!
- As title, set "Show my menu"
- For Menu Type, choose "CPCM"
- Save it
I have followed these steps myself, and you can see the result at blog.dianakoenraadt.nl
If this does not help, or if you have decided not to trace these steps, I want to at least thank you for reporting your troubles. If I can, I will use your feedback to make the plugin better.