Ok the issue with ModpackManager is that it needs to grab the info of curseforge.
Thats why the software asks you to “PreCache” on the intial startup. If you dont do that then it does not know any mods. So what you can do is.
Options > Repository > Open ModRepository > PreCache and load curseforge data.
Then wait until its done. (Make yourself a tea or something) And then it should able to find mods mods.
It wont find everything because not every mod is on curseforge and also not every version still exists on curseforge.
If you dont want to do this look up the links of the mods and type them in. (You can also click ignore mod so it wont ask you for links but modnames)
This problem will be solved in the next larger patch (still on its way) where precaching is mandetory and will be done on the instaltion of the software. But thats at least a month or two away before I am finishing it.
I hope it helps.