http://hcl.wcrevival.de/archive/treman1.zip
This is the download link to the TRE program you have. If you already have it, then great! If not, that's a straight download. You know the drill - right click and "Save as" and all that good stuff.
1) Extract the TREMAN zip to the WCSO/WCP directory
1) Open up the WCSO/WCP directory.
2) Right click on the TRE File and select "Open With..."
3) Add or select the TREMAN program, and make sure the "Open all files with this application" box is checked. This will make opening future TRE files easier.
4) Double click the TRE file you want to open - it should open in a dos window with a simple text menu.
5) Select the "Extract all files" option by pressing whatever number it is.
Repeat the process for ALL the TRE files in the game.
It might take a few minutes to get all the files out, after it's done there should be a number of different folders created in the WCSO/WCP directory.
If you're planning on building a campaign, with full briefings, and don't care about messing up your WCSO/WCP installation (You can always extract the TRE's again to overwrite any changes you make.) Alternatively, you can build your own TRE files (this is understandably, a bit more complicated) and then associate the EXE file to use your own files, but that gets messy.
If you just want to learn the basics, you should download the SOSIM application (
http://download.wcnews.com/files/so/sosim.zip) and write missions that overwrite the SIM missions. Later, when you've figured all that out, you can go to work writing a linked campaign series.