Take the Disneyland Dr. exit off I-5 and follow the signs. Easy-peasy!
I think there is a sign that says Mickey and Friends Parking Lot. The parking lot will be on your right, and it is before the DL Hotel, which is also on the right. Once you get off the freeway, you go down a block and over an overpass. I think the entrance is right after the overpass.
There are Informational signs on the freeway directing you to the parking. On I-5 it is important to look for these. When you arrive it is possible M&F parking will be closed and you could be directed to the Toy Story lot.
Exit at Disneyland Dr, then turn left at the bottom of the off-ramp.
Once you cross over Ball Rd (there is traffic light here), you're on Disney property. Stay to the left to enter M&F.
There is plenty of clear signage and it really is quite simple. Have a great trip!
It's extremely easy, just take the Disneyland exit and follow the signs, we've done that drive many times and never had problems, just make sure you watch for the signs.
BTW I've never seen a sign that says Mickey and Friends parking. The signs you will follow will say Theme Park Parking. As opposed to Hotel or DTD parking. It is very easy. As you enter the structure, I find that if you stay as far to the right as you can, those will usually be the shortest lines. Be aware that you can't see which lanes are open (with green x's) until you get pretty close to the parking booths. Sometimes some of the open lanes will be empty (people tend to follow other cars). As soon as you see one of the Green X's you may want to change to another lane.