Download the most recent version of flexunit-*.zip.
Unpack the zip into a location of your choice.
2. Include flexunit.swc in the compilation path
If
using Flex Builder, under Project, Properties, Flex Build Path, Library
Path, select the Add SWC option. Navigate to the file flexunit/bin/flexunit.swc
which is located in the archive unpacked above.
If using the command line, modify the mxmlc arguments to include
-library-path+=flexunit/bin/flexunit.swc adjusting the path as needed.