the metadata associated to the target is just a string, i.e. a blob of text;
in your case, this could be simply the URL where the 3D model is located, for example it might look similar to something like this:
Then, you will need to code something in C# to be able to download that model when your image target is recognized;
specifically, if you look at the code in CloudReco sample, you will find a script called CloudRecoEventHandler.cs (which is attached to the CloudRecognition object, which you can find in your scene view)
If you look at the code in CloudRecoEventHandler.cs, you will find a function called OnNewSearchResult; that's the function which is called by Vuforia when a target has been recognized on the Cloud, and that's the code where you can obtain the Metadata associated to the recognized target;
the metadata is in practice simply obtained with this code:
string metadata = targetSearchResult.MetaData;
For example in your case, the metadata will be a string like: "http://my_server/my_models/my_model_001.unitypackage" (assuming you had uploaded that metadata when you created the image target in the Cloud database)
Once you have that metadata, you can use such string to download that model from the server.
At this point you will need some code to perform such download operation; the best is to follow the tutorials in the Unity website about how to use (download) AssetBundles:
I hope this helps.
Note: other approaches might be possible as well, so you can also take different strategies if you like (mine is just a suggestion)