(See edit in first post)
Firstly, I made a small change to the xclip.sh file.
(This isn't related to your problem but may as well change)
To save dowloading again,make this edit..
echo $1 | tr -d '\n' | xclip
Save the file and close.
echo "$1" | tr -d '\n' | xclip
Check the scripts are executable ....
It works here with diodon in fact you don't even need a clipboard manager running for it to work.
chmod +x ~/.snippets-launcher/date.sh ~/.snippets-launcher/snippets.sh ~/.snippets-launcher/xclip.sh
The xclip.sh script is whats's run when you click on a snippet in the quicklist.
Run the command....
If "test snippet works" shows with a middle click paste then the xclip script is working.
~/.snippets-launcher/xclip.sh 'test snippet works'
If not, try sending to the ctrl+c/v of the clipboard.(right click copy/paste)
Use diodons menu item "clear" in between tests
Try also -selection primary (should send to middle click same as with no option)
echo "test snippet works" | tr -d '\n' | xclip -selection clipboard
Let me no the results and we'll go from there.