If you are installing the add-on not from official mozilla.org follow this steps.
1. Download the .xpi file (probably you already have it).
3. Menu Tools->Addons. It will open Add-ons Manager tab window.
4. Near to the top-right corner of the window you will see little button. While hovering mouse cursor over it you will see something like Tools for all add-ons. So, click on this button and choose 'Install add-on from file...'.
5. Select the .xpi file.
6. Click Open.
I hope it will help.
Here is general recipe.
First of all:
- Enable debugging in the add-on Options->Miscellanous.
- Make sure that your antivirus software doesn't block any operations.
- Reinstall the add-on.
- Restart Thunderbird.
- Restart your PC.
While trying the steps above don't forget to check the Thunderbird Error Console. It is highly probable that there will be some usefull messages. Also you may try to capture the full content of the Error Console on video and show it to me.
Regarding your problem specially. It looks that initialization of the add-on partially failed. The steps above may help to discover the reasons.
Please drop the message whether it helped to solve the problem or not.
Also I'm interested in the configuration of your computer: Thunderbird, OS, Antivirus versions; installed Thuderbird extensions; type of hardware;