Plasma 5

Compile from source. There was a PPA, but it's deprecated now because of the release of Kubuntu 15.04.
https://community.kde.org/Plasma/Building

That's still to hard for me. If this is the only way I'll have to wait until it comes from the trisquel servers one day. If it does at all...

It will be in the repos of Trisquel 8, not 7. Why not compile from source, though? It shouldn't be that hard.

https://launchpad.net/~neon/+archive/ubuntu/ppa?field.series_filter=trusty ?

Trust me - compiling is very easy. I'll give you a tip - check this command sudo apt-get build-dep "name of application to build"
mao!

Sounds easy but where do I have to run that command? I mean at what point?

Supose you have version (i'm making this up) 1.2 of the application named teletubbies in your repositories and you want the latest version for some reason, version 1.4.

When you build something usually the headaches are caused by the dependencies. it gives you errors because you miss dependencies. This is why that command for me is a must.
before starting to configure and make the first thing you type in your terminal is that command and that will install all the dependencies that you need in order to build something.
Once you have the dependencies usually the process is trivial:

./configure
make
sudo make install

but read always the install or readme file in the source code folder. it may vary.
for example - building minitube:

qmake
make

anyway, as you see it is no rocket science..

cheers!