Ten men accused of conspiring to topple the prime minister of the Caribbean island of Dominica have pleaded not guilty to the charges and will go to trial June 15. The 10 are charged with conspiring to invade Dominica and planning and preparing for that mission. They are also charged with federal firearms violations. The maximum penalty on conviction is a 50-year prison term and $38, 000 fines. The grand jury that indicted the men is continuing to investigate the plot, a nd more indictments are expected.