Syntax looks correct. Could be simplified to the following if you have auto self-cast enabled.
#showtooltip
/cast [@mouseover,help,nodead] [] Shadow Mend
or expanded to something more complex like this…
So the question then becomes, what key do you have it bound to? If you’re trying to cast it with a mouse click then you’ll need to install Clique or another click-casting addon as unit frames eat mouse clicks by default.