As it works with latex you probably do not have a layout file that tell
LyX how to use the leaflet class.
These are not that difficult to produce, and as leaflet seem to be quit
close to the standard article-class you only need to make a copy of the
article.layoutfile to fit your need.
All you (probably) have to do is to find where LyX has a file called
article.layout and make a copy, calling it leaflet.layout in the same
directory as article.layout.
Open leaflet.layout in your favorite texteditor and change second line
# \DeclareLaTeXClass{article}
to
# \DeclareLaTeXClass{leaflet}
run reconfigure in lyx and restart lyx and leaflet should appear. You
might use some ert and the preamble to tune the class, but most basic
functionalities should be there.
for more details check chapter 5. in the Customization.lyx that are
shipped with LyX.
Ingar
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