It should be installed in a folder (call it whatever you want as long as the name of the folder does not have spaces) in your FS2 root directory. Put there everything contained in the zipped archive you downloaded.
Open the mod.ini file you just extracted along with all the other files. The "secondrylist" line should have the names of the FSPort 3 and MediaVPs folder. Change their names to reflect the names of your FSPort 3 and Media VPs folder.
Now open the launcher, select your Caldinar Spear folder as a mod and, once ingame, don't forget to go to the campaign room and select the Cardinal Spear campaign
