To access any level hit the "~" key, which brings down the console. Then type: "world levelname", where levelname is one of the following:

01_ambush
02_quarters
03_mca_dock
04_shuttlebay
05_lz_minotaur
06_entrance
07_avernus1
08_airshafts
09_comm1
10_comm2
12_comm3
13_new_mca
14_maritropa1
15_depot
16_slumes
17_lost_cat
18_slumes
19_rendezvous
20_maritropa1
21_maritropa2
22_tram
24_cmc_sec1
25_cmc_sec2
26_tram
27_new_mca
28_airshipdock
29_airship
30_research
31_avernus1
32_MUSEUM
33_QUARTERS
34_spires
34_energy_plant
35_command
35_fortress
36_elevator
36_kato_center
37_gabriel