1. Make sure you have the spring lock tool fully inserted.
2. Keep pulling on the lines, but first make sure you remove the 6mm allen bolt holding the lines and the receiver/drier bracket, near the headlight (NOT the one securing the two lineset flanges together!) It's the one closest to the drier bottle.
3. I would pull on the lines in the engine area, don't mess with the plenum yet, it does take some effort, those O-rings hold it pretty tight.
4. Be positive that you have the tool properly inserted! You are actually EXPANDING the spring to allow the flared end of the evaporator pipe to slide past the spring, you are not compressing the spring.

If you purchased an aftermarket evaporator, the lines probably are not bent correctly. Once you have the new evap installed, it will be VERY difficult to line up the lineset in the engine compartment because they are very rigid. The alignment of the evap pipes must be VERY close, it is a tight fit and any misalignment will make it extremely difficult to install the O-rings/line set. I would recommend making a "gauge" to bend the lines of the new evaporator to match those on the removed unit. You could use a cardboard box, cut to the proper size, holding two edges of the box to two adjacent outside edges of the old evaporator. If you cut the depth correctly, you could mark the location of the pipes by punching a hole where they intersect with the box. But this is just where the ends of the pipe end up in relationship with the exterior of the evaporator, you also need to try to get the angle correct. Don't be afraid to bend the pipes of the new evaporator, just ensure you do not crimp, crush or distort the flared end where the O-rings seat. If you have a black plastic clip on one of the pipes of the old one it acts as a bracket to secure the pipe to the body of evaporator. I would pry it off the old one and cut off the tabs that get shoved in between the fins - you don't need them. Install it on the new one. This will help hold the pipe steady because you will need to push pretty hard when you try to secure the spring lock connection again. You will be very sorry if you do not take the time to get the pipes aligned as close as possible to the original unit PRIOR to installing it in the climate plenum. Minor misalignment can be corrected by bending the steel lines within the engine compartment but be aware this is easier said than done. Always use new O-rings, I lubed them up with a light coating of silicone grease.

When pushing the evaporator lines thru the firewall, be careful. You need to balance using enough force to push the lines thru the rubber grommet (lube it up), and push the heater core lines thru the firewall (aligning both at the same time!) and not bend or damage the flared ends. If you do damage the flared end, it will be difficult to get the spring lock to lock again. But you can't be a wuss about it either, it takes some force to do it. It might help to have a helper look from the firewall side to see how your alignment is before you push the plenum hard.

Always replace the receiver drier, my hose was seized to the bottle but was able to get it off when I removed the hose at the compressor and whacked the wrench with a 5lb hammer a few times.

Pull a vacuum on it when your done, ensure you have no leaks.