Hi FABJAC and welcome to the forum.
Please write in English here, as this is an international forum, so that as many people as possible can understand you.
The following is translated using Google Translate:
Yesterday I installed the livery file from the site, then I went to the game folder located on the file explorer .... From there I went to: data / scenery / models / aircraft and from there I added the two livery files . Wasn't that supposed to be done?
Scenery/Models is the folder used for static aircraft in the scenery, it is *not* the folder containing the actual "flyable" aircraft. For this, you need to go into Aircraft/(737-300 in this case) and extract the downloaded folder there in a way that the .xml and .png files of the folder are in Aircraft/737-300/Models/Liveries/. If you don't find the aircraft in this folder, this may be due to installing them from the launcher, which puts them in a different folder so that the aircrafts persist even if you install another version. This location is dependent on your OS.