I have managed to fix my problem:
"One possible error is that you did not explicitly select nullmac/nullrdc. By default, Contiki uses a duty-cycling MAC protocol, and that would create problems."
Anyhow, I had a peek inside of conitki-z1-main.c and I disabled the rime initialization as well as disabled the X-MAC set up for 802.15.4. This proved to be the solution. I had no need to manually calibrate the frequency synthesizer using STXCAL, as the poster in http://e2e.ti.com/support/low_power_rf/f/155/t/15914.aspx did.
