It's because you originally installed Installous from a different source. Since people can repack apps and put them on different repos, I have CONFLICTS set in the control file which will do exactly what it's doing to you right now which is to let you know there is a conflict with another package.
If you plan to use the one off our repo you can just continue to install via Cydia and it will take care of everything for you. Basically it's going to UNINSTALL the old Installous and it's dependencies and then it will INSTALL the one from our repo and make all the necessary configurations for you.
It's normally easier to just stick to one source this way you don't have crap all mixed and matched. Most people who have their own repo don't take the time to set their packages up right because they don't understand how all that stuff works in the control files. I actually know WTF all that stuff means in the control files so I'm able to better configure and pack all the packages that go on the repo and am very meticulous about keeping everything in order. I take pride in making sure that users not only find all the apps they want, but that they are actually SAFE to install on their device as well
BTW, now that I have helped you with your issue perhaps you could send me some Cheese Coneys or a 3-Way from your local Skyline out there in Kentucky