Open up Notepad (or any text file program) and copy this to it:
save and rename the file autoexec.cfg, place in base folder at the root of your brink game folder and your done!net_allowCheats "1"
g_infiniteAmmo "1"
g_infinitePower "1"
g_instantRespawn "1"
g_noGrenadeCooldown "1"
Infinite ammo is self-explanatory, infinite power means the supply meter never depletes. Instant respawn mean you no longer have to wait to respawn at base. No grenade cooldown means you can spam grenades as much as you want.
Here are a ton of different cvars you can try.
Sadly the console commands don't seem to work with this method.
Edit: Sample file uploaded for anyone having trouble with this.