As the work with semantics and services grows more ambitious in the Semantic Web community, there is an increasing appreciation on the need for principled approaches for representing and reasoning under uncertainty. Reacting to this trend, the World Wide Web Consortium (W3C) has recently created the Uncertainty Reasoning for the World Wide Web Incubator Group (URW3-XG) to better define the challenge of reasoning with and representing uncertain information available through the World Wide Web and related WWW technologies. In according to the URW3-XG effort this Chapter presents the implementation of a graphical user interface (GUI) for building probabilistic ontologies, an application programming interface (API) for saving and loading these ontologies and a grammar proposal to specify formulas for creating conditional probabilistic tables dynamically. The language used for building probabilistic ontologies is Probabilistic OWL (PR-OWL), an extension for OWL based on Multi-Entity Bayesian Network (MEBN). The GUI, API, and the compiler for the proposed grammar were implemented into UnBBayes-MEBN, an open source, Java-based application that provides an easy way for building probabilistic ontologies and reasoning based on the PR-OWL/MEBN framework.