To access a File object that refers to the currently executing .spln file, use

theSPLNFile = app.callExtension(0x90B6C,10015);

or, in more 'wordy' fashion:

const 0x90B6C = IID_IACTIVEPAGEITEMSCRIPTUTILITIESEXTENSION;
const kOpcode_SPLNFile = 10015;
...
theSPLNFile = app.callExtension(IACTIVEPAGEITEMSCRIPTUTILITIESEXTENSION,kOpcode_SPLNFile);