Problems Importing and Exporting Presets in Lightroom Classic CC v7.3.1

1º - When i click on the + sign of the Presets Panel, there is no Export Presets button

In LR 7.3, you export a preset simply by making a copy of its .xmp file. You can easily find the .xmp file by right-clicking the preset in the Presets panel and doing Show In Finder / Explorer.

2º - When i click on the + sign of the Presets Panel, and choose the Import Preset option, i choose a xml preset file and click ok, the preset just don´t appear in my Presets Panel, like it was not imported.

LR 7.3.1 still has a number of bugs with presets, including that it often doesn't sort them correctly.  Make sure you examine the entire list of presets in each group (folder) of presets.

If you still can't find the imported preset, upload the .xmp file to Dropbox or similar and post the sharing link here.  We can take a look to see if it's a valid preset or if trips over one of the bugs that still exists with preset names.

To work around the sorting bug, see the free Fix Presets plugin.

3º - When i try to create a new preset folder (ctrl + Alt + n), it just is not created. (the only way i could create a new preset folder was by this option while creating a new preset, in new develop preset window).

Adobe has indicated that will be fixed in the next release, likely due any day now. You've already identified the workaround.