Install the plugin by downloading the gplugin file and dragging it in to Geneious or use the plugin manager in Geneious (under Tools - Plugins in the menu).
Once installed, run the plugin by selecting a nucleotide sequence and choosing Annotate & Predict - Find Genes with Augustus from the menu. Results are saved as annotations on your sequence.
Mario Stanke, Rasmus Steinkamp, Stephan Waack and Burkhard Morgenstern (2004) "AUGUSTUS: a web server for gene finding in eukaryotes" Nucleic Acids Research, Vol. 32, W309-W312