Lightroom Mobile requires that you have a active subscription to the Creative Cloud Photographers package. (includes PS CC 2014 and LR for $9.99 pm.)
If you existing LR 5 is the perpetual version with a license key then you will need to subscribe to the package above.
Ah, okay! Thanks!
Also, how many computers can I install LR on? I have it on my laptop and desktop currently, but am contemplating a new laptop soon? I am not subscribed to the CC package; I only have the LR 5 key
Your Lightroom key is valid for both Mac & or PC and is a single user/owner license.
Two active installations are allowed, with the understanding that only one is in use at any point in time.
You can install/uninstall as many time as necessary. Just use the key to activate.
To clarify, a subscription to the PS+LR Photography Plan has a different LR license than your current serial-number-licensed LR, and each license-type can be installed on 2 computers, so you could have LR on up to four computers, two of which can be running the CC licensed version of LR along with the CC-licensed version of PS, and two running the serial-number-licensed LR without PS-CC.
If you have your catalog and photos on an external drive that you can swap between the various computers, then you could use any of the up-to-four to work with your photos. You could also have as separate catalog on each of the up-to-four computers, but then syncing would get complicated if you try to do that on more than one LR catalog, I think, though I could be wrong, because I only use LR Mobile with one catalog, currently.