The author of the linked article probably had a modified bash profile (like explained by the user user3439894 in the comments to your question) or had problems to copy and paste the commands.
Use the following instead:
cd ~/Library/Application\ Support
cp -r Mendeley\ Desktop/ ~/Dropbox/Mendeley/db/
mv Mendeley\ Desktop/ _backup_Mendeley_Desktop/
ln -s ~/Dropbox/Mendeley/db/Mendeley\ Desktop .
Please don't forget to include the dot
. at the end of the last command. The last line means: create a soft link of the folder "~/Dropbox/Mendeley/db/Mendeley Desktop" at your current working directory (which is "~/Library/Application Support").
cd ~/Library/Application\ Support: change the working directory to "/Users/YourUserFolder/Library/Application Support".
cp -r Mendeley\ Desktop/ ~/Dropbox/Mendeley/db/: copy the folder (including the content) "Mendeley Desktop" into the folder "~/Dropbox/Mendeley/db/"
mv Mendeley\ Desktop/ _backup_Mendeley_Desktop/: rename (move/backup) the folder "Mendeley Desktop" to "_backup_Mendeley_Desktop".
ln -s ~/Dropbox/Mendeley/db/Mendeley\ Desktop .: create a soft link of the folder "~/Dropbox/Mendeley/db/Mendeley Desktop" at your current working directory.