First off, Miguel, that's "trace", not "tracer". The USM-281A was the Hewlett-Packard version of that series; Fairchild, Tektronix and DuMont made the 281B, C and D respectively as I recall -- well, I'm abolutely positive the C was Tek's. Anyhow, that's just a little trivia.
You have two plug-ins in this scope, a vertical preamp and the timebase. Make sure that the timebase trigger mode is in the "AUTO" vs. "NORMAL" or "SINGLE SWEEP" mode and that the TIME/DIV is set to around 1ms/DIV. On the vertical preamp plug-in, simply set it for CHANNEL 1 only with the input coupling switches set to GND.
With this, you should have a solid horizontal sweep which you can move a little left and right with the horizontal position control (on the timebase) and vertically with the vertical position control (on the preamp).
Let's see where you are are now.
Dean