What component should I use to represent a solenoid valve in multisim
If you aren't doing anything super-fancy, you can probably safely assume that a 12V 500mA solenoid has an impedance of 12v / 500mA = 24 Ohms.

If it's going to be switched relatively slowly (1/sec is pretty slow), you can assume a 24 Ohm resistor and just remember to put a backwards-facing diode in parallel with the coil to protect your circuitry from the collapsing magnetic field's current when you get ready to build it.