Put spool on, hold on with threaded nut, push and twist on spool cover.
If you're missing parts you can get the kit from ALM Manucturing
part number ALM BD030
http://www.almmanufacturing.com/index.php/products/garden-machine-parts/grass-trimmers/black-&-decker-grass-trimmers/item/177-alm-bd030-spool-housing
Richard, April 2015