You should only open a BGL file if you are legally entitled to do so.
FSX Planner works with XML files. You can either open an existing airport XML file, or you can decompile a BGL file into an XML file and use that as the starting point for your airport. If you deompile a BGL file you should then save it as an XML file and continue to use the XML file as the source code for your airport.
Think of it in the same way as writing application code. The XML file is the source code which you use to make any modifications to the airports. You then compile the XML file into a BGL file when you want to test or release the airport.
Locating the correct bgl file to convert
If you want to edit a default Flight Simulator airport then you need to locate the bgl file that contains the airport. There are hundreds of bgl files within Flight Simulator X, so FSX Planner helps you determine which one contains the airport you are interested in editing.
Selecting File -> Find Airport will display a dialog where you can search for the airport by entering the ICAO code or the airport name. Any partial or full matches will be returned. You can then double click on the appropriate airport. The BGL file containing that airport will be decompiled into an XML file that FSX Planner will then display.
Please note that this XML file exists only in the computer's memory. You will need to save the airport to XML in order to create a source XML file for the airport. Then in the future, you should open that XML file to work on your airport. FSX Planner does not make any changes to existing BGL files. Once you are ready to put your airport into flight simulator, you compile the XML file into a BGL file using the Compile -> Compile To BGL File option.
The first time you search for an airport, you will need to perform a one time full scan of your flight simulator install in order for FSX Planner to generate a summary file of the airports.
Opening xml files that contain multiple airports
An xml file can contain more than one airport. When converting a bgl file to an xml file it will most likely contain many airports. When you open the xml file, either directly or by converting a bgl file, FSX Planner will show you a list of all the airports available in that file. You can select one or more airports to open from this list by ticking the checkbox next to the airport's name.
After opening the airport it is highly recommended that you perform a Save As to create an xml file that contains only one airport. From this point on you should use the newly created xml file as the airport source file. It is not good practice to save a file as a bgl file and then decompile it everytime you want to open it.
We hope that this tutorial has explained how to find airports and open bgl files in FSX Planner. FSX Planner contains many more features, and we hope that you are able to explore and benefit from them.