ctrl+l inserts λ
ctrl+d inserts ≜
ctrl+enter does one step of evaluation or define/undefine
ctrl+r rewrites stuff and replaces names defined elsewhere with their definitions
ctrl+shift+enter does lots of evaluation
right-click and Show definitions to list the available definitions

e.g.
put cursor on line below and ctrl+enter
0 ≜ λf.λx.x

put cursor on line below and ctrl+enter
S ≜ λn.λf.λx.f (n f x)

put cursor on line below and ctrl+enter
2 ≜ S (S 0)

put cursor on line below and ctrl+enter
+ ≜ λa.λb.a S b

put cursor on line below and ctrl+enter
6 ≜ + 2 (+ 2 2)

put cursor on line below and ctrl+r, then ctrl+shift+enter
+ 6 6