That component was added in 8.0.1086, so you should be able to access it. It looks like you'll need to manually add it by right-clicking the Toolbox and choosing "Choose Items". Then under the .NET Framework Components tab, you can search for JsonUtils and add it.
Do you have any examples on how to use these. I have process I am working on that requires me to call a REST API and store the results as useable variables later in our OpenSpan Process. I have seen many posts where users are using C# to do this, but If that restClient works as a better method we would like to use it instead.