@leonards1 Splunk's Custom Visualization API was introduced with 6.4. However, it got changed with 6.5 onward. So, the Custom Visualizations built on previous API may not be compatible with the latest versions. You might have to contact the developer or build your own using latest Custom Visualization API.
Also, Tag Cloud visualization is also built in to Splunk (6.4 onward) along with other default visualizations. However, it does not have too many customization options as compared to Word Cloud. Get Splunk Dashboard Examples App from Splunkbase to see some of Tag Cloud examples.