In order to monitor stocks and other securities' prices in Medved Trader you have to create either a portfolio or a watch list containing the symbols for the stocks/equities, assign it a data source and Start it.
Both a portfolio and a watch list are basically a list of symbols for the equities you'd like to monitor. The difference between the two is that a watch list is just that - a list of symbols. A portfolio also contains other data for each symbol - such as quantity bought or sold short, price at which it was bought, etc.
More on this topic at: Portfolios and Watch Lists.