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 your reads and reference sequence then clicking on Align/Assemble - Map to Reference in the toolbar. TopHat is available as an option in the Algorithm drop-down menu.
Trapnell C, Pachter L, Salzberg SL, TopHat: discovering splice junctions with RNA-Seq. Bioinformatics 2009 25(9):1105-1111.