You have 2 elevators that will help you to get to the apartment in 10 minutes after you leave the metro. It's called ELEVADOR DO CASTELO .
You can take the metro at the airport or train station and come out at BAIXA-CHIADO station ( green and blue line). It has 2 exists and you should follow the signs saying RUA DO CRUCIFIXO . When you came out walk straight to the end of the street and you will find and elevator (first elevator ) that will take you to RUA DA MADALENA . Turn left walk up for about 50m and you will have on your right a new building with a sign saying Pingo Doce and there you will find another elevator. Take it all the way up and turn left at Costa do Castelo the street of your apartment. Walk to number 51 and you have arrived. Please ring the bell on 1E.
This instructions may seem complicated but it's quite intuitive when you are there. If you are ok to walk then you enjoy it.