You won’t be able to access it via IP address, as that address will be the internal IP address of the MCM as assigned by the router in your office (192.168.x.x or possibly 10.10.x.x or somesuch depending on your router set-up).
What you need to do is enable cloud access on your MCM (see section 4 of the MCM user manual, page 22 onwards) and then use that activation code along with the relevant WD app for the device you’re trying to use remotely to access the MCM to set up the remote access. You also need to ensure that cloud access is turned on (under the settings menu on the MCM dashboard, go to the general settings tab and make sure the “cloud access” switch is set to on (see page 108 of the manual). This is presuming that your office router is set up to allow uPNP connection and port-forwarding to allow the connection in through it from outside to the MCM - if it doesn’t work you’ll need to talk to your office IT people about whether it could be permissable to set it up or not.
The activation code only needs to be done once to authorise the given user (and associated email address) to have the remote access. After that they can use the apps to remote-connect to any shares which they would normally have permission to see if they were directly connecting to the MCM on the local network.
Note that this will only allow access via the WD apps, it can’t be used (for example) to map a drive under Windows Explorer or anything like that. If you want to work on files, in most cases you will need to download a copy via the app, work on that local copy and then upload again via the app. It depends a little bit on the file-type involved, but it is generally the case.
An alternative way to do it is to arrange for a remote access onto your office network itself (a VPN log-in or something like that), at which point your MCM will be available normally as you will in effect be logged into the network as if you were physically sat in the office and connected directly.