Hamilton, M., R. Russo, AND R. Thurston. TRIMMED SPEARMAN-KARBER METHOD FOR ESTIMATING MEDIAN LETHAL CONCENTRATIONS IN TOXICITY BIOASSAYS. U.S. Environmental Protection Agency, Washington, D.C., EPA/600/J-77/178 (NTIS PB81191918), 1977.
There are many softwares for probit analysis --- Polo Plus, SPSS, R. If you want a good statistical software (with probit analysis) on Mac, try Statplus or GraphPad.