In the old launcher, I can edit the modlist from a modpack. How can I do it in the curse launcher?
First, there used to be a "lock" setting on "Profile Options". This appears to have been removed recently, so it looks like you won't need to do that anymore.
To Disable/Completely remove mods a from a pack:
1). Right Click the Icon for the loaded mod pack you want to change
2). When the list of the mods in the pack currently installed on your system loads, there's a box with a check mark on the left of each mod. If it is Green, the pack is enabled. Click the box to turn it Red and the pack is disabled.
3). Conversely, when you are sure you don't want the mod at all there's a Red "X" on the far right of each pack... Clicking this will remove the mod completely.
To add mods:
1). There is a button near the top of the window labelled "Get More Content". Click it.
2). A new list of all the usable mods appears, based on the versions of Minecraft and Forge the profile lists. (I only saw one mod listed incorrectly in early beta). if you see an interesting mod hover your pointing device/digit over the mod icon... If you see a black area with "Installed" printed in it, then the mod is already in the pack, possibly disabled. If a green button appears saying "Install" and you want the pack, click it. The mod loads and installs immediately.
That's it.
Note: If you are changing a supported pack, say FtB Unstable 1.9, you can make a copy of the original pack by Right Clicking the Pack icon in the first Minecraft screen and selecting "Duplicate Profile". This creates a completely separate installation of the pack that you can rename. The same Drop Menu also contains the selection "Delete Profile", which completely removes the selection from the system.
I create a copy of a non-custom pack and then modify it since "Official" packs also come with custom configurations that I don't want to disrupt by removing a key mod. If I make a mistake it's easier to delete the my error and make a new working copy than it is to download the whole pack again.