IMFD isn't capable of planning slingshots (won't help find the window) but it is capable of performing the flight after it's been planned. The important info is the exact date of each flyby, and the altitude of each flyby.
For instance, let's say I'm doing an EJS (Earth to Jupiter to Saturn) sling. The first leg I will set Target Intercept using the Earth ejection and Jupiter arrival dates indicated by the plan. When I get close to Jupiter (but still outside it's SOI) I will use Planet Approach using the swingby date and altitude from my plan. Once inside Jupiter's SOI, I will set up the second leg in Target Intercept (will use Jupiter for Src, Saturn for TGT) using my PeT (from IMFD's Map program) for the TEj, and the TIn (arrival time) from the plan, then use the slingshot program for the burn. It's not the best - IMFD will want to align with the ejection plane AND add any velocity all in one burn. However, the alignment should be made as far from Jupiter as possible, while any velocity should be added at Pe. With a Jupiter sling this usually isn't a problem, because you won't be adding velocity in the burn (you will get more than enough additional velocity from the gravity assist) so you can just hit the AB right away to align with the ejection plane.
There is an error in the IMFD Full Manual (mentioned above) due to a communication error between Mark and I. In the manual, the second leg is set up the same way as the first leg, which is incorrect (and results in Mark making a HUGE correction later).
The proper way to set up the second leg is to have Target Intercept open in one MFD, and Map in the other. The SRC is whatever planet you are approaching, the TGT is whatever planet is next on your trip. Set the TEj in TI to match the PeT shown in Map. Now replace Map with Slingshot. Now we will adjust the TIn in Target Intercept while watching the Tot dV shown in Slingshot . Just ignore any dV shown in Target Intercept and try to get the lowest Tot dV shown in slingshot. Then, watch the dV shown in slingshot - if it's increasing burn immediately. If it's decreasing, wait until it stops decreasing and starts to increase, then burn. Sometimes a second burn is useful on the way out - at least halfway from Pe to leaving the SOI - to "clean up", since the planet's moons may have affected your course during the sling (common around Jupiter and Saturn).
