To do this you need to use the very slow and tedious long press on each track you wish to add to the play list.

The key to remember is that you only have the 'Now playing' list available to you when creating playlists in the UI. Try these steps:

Preconditions: you have written down all the tracks you wish to add in the correct order to ensure you don't mix things up.

1. Launch the media player
2. Go to 'Now playing' and clear any current tracks (Menu->Clear current playlist). You should now be returned to the 'Music' view.
3. Find the track you wish to add.
4. Tap and hold over the track until the context menu is shown.
5. Select the 'Add to current play list', tick off your list.
6. Repeat steps 3-5 for the next track. If at any time you add a track in the wrong order or as will happen a tap and hold gets interpreted as a tap so that playback of all tracks in the current view kicks off, you are welcome curse profusely before returning to step 2. Once you get bored or too angry to continue move onto step 7.
7. Once you have been through that pain go to the now playing view and admire your work, just don't forget to save the playlist (Menu->Save playlist) so you don't have to go through all that pain again (for this playlist)

I have yet to find a truly great playlist editor but the maemo 5 media player takes a close second as the worst.