File - Import - Flex Project
Then select your project folder.
First of all do what mr. M said.
and then try to open main mxml file in editor and press green "Run" button on the top panel,also you might try to right-click on that mxml file in project explorer and execute two items:
"Set as Default application"
The first thing I tried is to import application as a project.
But Flex Builder has its own opinion about this and answers:
"The folder does not contain a valid Flex Builder Project."
This is why I listed all the files and folders I have.
You could try to create a new project then import the files
File - New - Flex Project
Then I think there's an option on that wizard to select existing source files.
Is your existing project under version control...? Could be that the required files (I think .actionScriptProperties and .flexProperties) were ignored, in which case, get a copy of them from another devlepor, then change them (folders, etc.) to accomodate your machine.
I created a new project, copied all existing project structure into new "src" folder and renamed old main .mxml file with the new project name. I guess it is not correct way to do it but at least now I can try to compile it.
Thank you all for your help.