Hi Alvaro,
Platesolve3 requires the catalogs to be correct and in the correct location on your PC.  Here is the writeup on setting up Platesolve3 catalogs (from the readme file that comes with each SiTech install file.

Don W

PlateSolve3 Catalog locations!
This is important if you want to use PlateSolve3, (I highly recommend it).
PlateSolve3 (PS3), does very fast all sky plate solves (Thanks Dave Rowe!)

The Star catalog for PS3 is about 2.1 GBytes zipped up.  
You can try to download it here:
http://siderealtechnology.com/PlateSolve3Catalogs.zip
If that doesn't work, we're trying to set up a public FTP, when we get it working, we'll announce it on the list.
Anyway, you need to unzip it, and it has to be in an exact location.

In the following paragraphs, we're trying to say where the folders MUST be. On most U.S. computers,
the folders are here:
C:\ProgramData\SiTech\,
But your mileage may vary.  Sometimes it's:
C:\users\AllUsers.windows\application data,
and there's a variety of others where windows puts this folder.
You have to figure where the data files for SiTechEXE is, then going up one level.
In the following discussion, I'm going to call it:
C:\YourProgramDataPathWhateverThatMayBe\

You can find this SiTech Data folder by running SiTechExe, clicking on the Config tab,
then Click on Change Config.  Now look at the path in the top textbox labeled "Folder for SiTech.cfg"
Copy everything from the C: up to but not including the SiTechExe, then paste it into a MyComputer browser.
That's where the catalog files MUST go.

The unzipped files should go here:
C:\YourProgramDataPathWhateverThatMayBe\SiTech\PlateSolve3Catalogs

When you unzip the files, sometimes it will make a folder under a folder, like this.
C:\YourProgramDataPathWhateverThatMayBe\SiTech\PlateSolve3Catalogs\PlateSolve3Catalogs   WRONG!!!
This is Wrong, and won't work

Again, the files must be here:
C:\YourProgramDataPathWhateverThatMayBe\SiTech\PlateSolve3Catalogs
In the above folder, there are 3 sub folders:
Orca
UC4
UC4Mag14