At the railway station ask for Dluga Street - it will take you about 7 minutes to get there.
When you're at Filipa ask for Dluga. If you'd like to take a tram from the station (it will take you about 5-7 minutes to get to the hostel).
Just take tram No. 3, 5, 19 - and get off at the 2nd stop (D³uga') - and after right part you see blue ASTON HOSTEL logo.