Sorry, but (as its author), I don't rely on IDEs and stuff. Just plain ol' Scratchbox and gedit.
I assume you have Scratchbox installed due to your mention of ESbox, so try this:

1. In Scratchbox, cd to wherever you've unpacked the source. (If not, add Extras-devel to your sources.list in Scratchbox and type "apt-get update && apt-get source simple-brightness-applet")
2. Run "apt-get build-dep simple-brightness-applet" to get whatever packages it needs to build.
3. Issue a "dpkg-buildpackage -rfakeroot" and hope that it works...

The above should get you a deb. Edit the source if required and keep on building.

P.S. On subsequent builds, you can pass "-nc" to dpkg-buildpackage to have it speed up the process a bit by not having it clean up the files first before rebuilding.