open the contents of the flan mods zip file and put this in one window on the side of the screen then open up minecraft.jar using apps such-as 7-zip or win-zip and put that on the other side of the screen
you have to manually go through and find every file and folder that as been added from when you transfered the files from the flan mod and you have to delete them
if this doesnt get it working again then you will have to do Force Update
open up Minecraft and then click on the options and then find the force update button
ok so the first way will manually fix the problem and will restore it to before the flan mod was installed
and the second way will wipe all of your mods completely and update it to the latest version
oh one last thing minecraft is now at 1.6.2 so when you force update it will update it to 1.6.2 and not 1.5.2
so if that happens then you can just go into the user/%appdata%/roaming/.minecraft/bin folder, remove minecraft jar file and then download a minecraft.jar snapshot from the link in the sources below
then place the minecraft.jar file from the download into user/%appdata%/roaming/.minecraft/bin folder
and finally when you load up minecraft it will say do you want to update just say no to that.
edit: ok i read your question fully and heres my answer if it says that a certain mod is not installed then you have to find the download for 1.5.2 and stick it in the core mods folder or the mods folder and then forge should load it up fine.
edit: the only one available is the 1.2.5 version one I cant find one for 1.5.2 but im also thinking itl work in tekkit classic because that runs minecraft version 1.2.5
so stick it in the tekkit coremods folder and see what happens