This can be done by clicking on the settings tab, turning on the "Connect to Runkeeper" using the slider and turning on "Runkeeper Autodownload" using the slider. Runkeeper will be queried every launch of Simple Lift Log and download any new activities that m ay be there. Make sure that "Connect to Runkeeper" is enabled first.
Unfortunately, unlike running activities, Runkeeper does not seem to allow modifications to strength activities once they are already posted. As such, any modifications you make to these workouts will stay on your device only.
Why did the completion date change after posting to Runkeeper?
Runkeeper does not appear to allow specifying the time and date that a strength activity occurred. Instead, it uses whatever the time and date is when the activity is posted. The time and date of the activity is modified locally to match.
Misc
What exercise types does auto-complete recognize?
On initial download, Simple Lift Log will recognize the name and muscle group for all of Runkeeper's primary types. Any types you wish to add can be done by selecting a muscle group after entering your exercise's name.
Why aren't my lifts being counted toward the total monthly lifted?
Only completed sets count toward the total amount lifted. Tap on the set to indicate that it is complete and a checkmark will appear next to it. After this, it will be considered a complete set and show up on the monthly stats.
Why is "post" greyed out on the workout creation screen?
Post will only be enabled if "Connect to Runkeeper" is enabled in the settings and the activity has not been previously posted to Runkeeper. If you would like to add exercises to the routine and post it, you can copy it from the workout selection screen and edit and post the new workout. This is because Runkeeper does not accept updates to strength activities, so each activity can only be posted once.