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 sequencing reads and clicking on Align/Assemble - De Novo Assemble in the toolbar. Velvet is available under the Algorithm drop-down option.
Velvet: algorithms for de novo short read assembly using de Bruijn graphs. D.R. Zerbino and E. Birney. Genome Research 18:821-829.