Most of the time, Teleport: AddOns should work flawlessly. However, if things go wrong, this is the page to visit!
This can happen if your World of Warcraft installation is in a place you need administrator access to. To solve this, we need to allow your account to write to the World of Warcraft installation folder.
Note: Do do this, you need an administrator's user name and password. If there's only one user on your machine, it will always be an administrator.
This can happen if a machine loses its internet connection or crashes during a sync, or if another machine is genuinely using syncing to the same sync location at the same time as you're attempting.
If you're sure no other machines are using your sync location at the exact time you see this error, you can click the "Unlock" button in this warning to forcefully unlock the sync location so you can use it.
If automatic unlocking fails, make sure you have a working internet connection (for online sync locations), are correctly signed in, and have write-access to the sync location. If this still doesn't work, find your sync file (in Dropbox, it's inside the "TeleportAddOns" folder on your Dropbox), right-click it in the Finder and choose "Show Package Contents...", then manually delete the "TeleportAddOnsSyncLock" file you see.