Compaq often puts a sticker on the bottom side of the tower with all the motherboard specs for that particular unit. Sometimes they put the sticker inside the case cover.
Most PC133 has a feature called SPD -- Serial Presence Detect. In some CMOS setups, you may need to change the default BIOS settings to recognize SPD if you want to run PC133 in a PC100 slot. Getting into Compaq CMOS setup can be a bit tricky.
Is it possible to test with a stick of PC100 instead of PC133?? Even if you get the PC133 to work, it's probably only going to run at PC100 anyway. What are the BIOS settings for your CPU bus speed?