I have an older inside central air/heat unit. I replaced the outside compressor/condensor unit. I have two low voltage wires comuing from the outside unit red/yellow. The yellow is spliced into the yellow wire gong to the thermostat. I don't know where the red wire goes. The compressor fan won't come on.

Are you licensed to do this work? People shouldnot help you in this situation. You may get hurt

See if you have 24 volts at outside contactor, may just be a bad contactor?
99% of the time the low voltage control wiring goes from thermostat to air handler and air handler to compressor, what wires did you have at the air handler when you installed it?
You need 24 volts out on compressor control wires when set to cool.

One wire goes to y(yellow) and the other wire goes to c(common). So in your case, the yellow is already spliced to the y and the red would go to c.

One wire goes to y(yellow) and the other wire goes to c(common). So in your case, the yellow is already spliced to the y and the red would go to c.
I don't have any connection marked c

Then the neutral on the 24v side of the transformer.

then the neutral on the 24v side of the transformer.

There is already a red wire hooked up to the neutral going to the thermostat when I hook the red wire coming from the compressor the blower runs constantly but the compressor still doesn't come on.

See if you have 24 volts at outside contactor, may just be a bad contactor?
99% of the time the low voltage control wiring goes from thermostat to air handler and air handler to compressor, what wires did you have at the air handler when you installed it?
You need 24 volts out on compressor control wires when set to cool.

1 side side of the secondary of the transformer goes to all contactors and relays(Fan, compressor, blower).
The other secondary wire goes to the thermostat common, then when set to cool, that hot/common will connect to the Fan and compressor control wires, blower relay is in air handler and compressor relay is in compressor unit outside.