Ok, so you got a pretty big project 
One of the files is missing the namespace System. Do you have the code for all the project files?
This is what is happening:
Code:
int x = 0; // error at this line, what is int?
Code:
using System; // System defines int
int x = 0; // this will work, int is defined
This is the same error you'd get if you do this:
Code:
bar baz = new bar(); // what is bar? it is not defined!
Code:
File foo.cs
namespace foo {
class bar { }
}
another_file.cs
using Foo; // now, you can use bar() and it will be the same as foo.bar()
bar baz = new bar(); // this works!
You could also change every single int to System.int, but it is much easier and better coding to use the namespace system.