M401? I started on that. I bought 2 other models within 4 weeks.
You take the cart off and stick a small unbent paper clip in the mouth hole to push up the tiny juice cup. You add some juice to the cup (just enough to look damp), make sure the stuffing is at the top edge of the juice cup, then get the juice cup back down into the mouthpiece without pressing on the stuffing (use paper clip against the inner wall of the mouthpiece and press down on juice cup edge, or use the cart plug).
It's a bit better with blue foam stuffing - I cut down the the small pieces for 510 at strictlyejuice.com (search there for foam). Cheap - under $1 for 10. I cut the foam about 3/4 the inner length of the juice cup and a tiny tiny bit wider - just enough to help it stay in place but not so it presses closed the holes in the foam). And taper the inner end a bit. Fill juice cup 1/2 way with juice. Press the foam, tapered end first, down into the liquid to wet it (use the paper clip), Reposition the foam at the top edge and top off with 1-3 drops - just to look damp, not a puddle. While vaping, top off as needed. When topping off does not work, do a full fill.
The carts are a pain in the a. Nice looking, nice flavor, but pain in the a. If you search around you MIGHT find some old CE2 cartomizers for it - those need a syringe to fill and will probably be a pain too since they have been improved since then. As for cart and atomizer? Carts suck, they will dump juice without warning. Most people use cartomizers,and to have access to lots of cartomizers in a slim ecig, you either need to go Joye 510 (plenty of blank cartos though Boge are most popular) or KR808D-1 (Vapor4Life, SmokelessImage - prefilled and blank cartos) and I recommend manual batteries on either to help prevent killing a battery if a cartomizer leaks a bit.
If you want something that looks like a cigarette to start with, get Joye 510 manual in white with a red/orage LED, and get the Boge tan or white cartomizers (standard resistance only!) at NiteLiteVapor.us. Don't buy a clone 510 - some are crappy and one is reverse polarity and fries real 510's with its chargers. There is a nice roomy cig-pack-look PCC for the short 180mah 510's at NoSmokeVA.com
(note - stuffing-type cartos will NOT dump their contents like carts do. Cartos occasionally leak a bit onto the battery. If you clean them up well after adding juice you should not get juice in your mouth)