You need to load up a master in order to see the contents. When you first open the Construction Set nothing is loaded, you need to choose what you want to load. Almost everything requires the Oblivion.esm to function so here is how you 'activate' it.
1. Open Construction Set
2. Go to File > Data > Double click Oblivion.esm (put x in the box) > Click OK
3. Prepare to get lost in here as much as you do in the game itself! It's capabilities are near endless.
Now if you want to alter a specific .esp file you need to 'Set it as Active' To do this follow this example.
1. Open Construction Set
2. Go to File > Data > Double click the .esp file you wish to alter, i.e. (MyMod.esp)
3. Click 'Set as Active File (Notice in Status it now says Active instead of Plugin)
4. Click Ok and wait for everything to load.
5. Edit away! Just remember to save your changes if you want to make them permanent. Just click File > Save and it will automatically overwrite the changes to MyMod.esp for example.
Just remember to make backups of any ESP you decide to alter or just mess around with. This way if you screw something up you have a backup of the original to revert back to.
Hope that helps.