Enzino,
For your specific case: it seems you have an expired MEP.
Is this on the front of your license?
If so, you have valid MEL, but not current (in the FAA’s language). The verification letter should include this, and a 61.75 certificate should indicate all privileges (except Instrument) at the Private level.
You can add Instrument Airplane by passing a shortened FAA knowledge test. Do this before applying for your 61.75 certificate.
The FAA certificate is a proper license in its own respect, as long as your foreign license is not surrendered, suspended, revoked or expired.
You do not need to be current on your foreign license to use the FAA privileges. But you must have an FAA Flight Review (or equivalent Wings credit), a valid medical (FAA or foreign), and if carrying passengers 3 takeoff & landings in 90 days.
The Flight Review will include 1-hour of ground, that should concentrate on areas appropriate to your level (ie US procedures etc).
I suggest the ASA FAR/AIM app for iPhone & iPad, as well as the ASA apps for Instrument & Private. The former will prepare you for the Instrument for Foreign Pilots exam.