Using it in LyX depends on whether it's supported by LyX. If it is, then all you have to do is click Edit->Reconfigure, then restart LyX and you can use it just like any other document class. If it isn't, then you will need to put LaTeX commands in your Preamble (in LyX 1.3.* you get there from Layout->Document). If there's any documnetation with the package, read it and see what it asks you to do. If not, try putting
\usepackage{parallel.sty}
in the preamble and see what happens.
I suppose an error ... :-)
Herbert
