IIRC the MagicSIM needs to be put in 007 mode first by using "any" GSM phone supporting STK. I.e. you will find in the phone menu anywhere (depending on phone model) an entry where to configure the SIM adpter. May it be under STK or Extras or SIM menu or or or.

If you put here your model (not the N900 of course) you should find lots of helpful hints.

As the N900 does not support the STK, SIM-Switcher relies to have the adapter set up to use 007 mode.
Then you need to set these switching codes in the config files.