You have done a repeated measures analysis in SPSS and find you have an interaction. In order to interpret this interaction you can analyse components of the interaction known as simple effects. Analysis of simple effects in a repeated measures design is only available using syntax. This syntax is described and illustrated here and is to be added on here. Field (2013, p.531) also illustrates this same approach (the /EMMEANS subcommand).

Simple effects for a between subjects analysis may be performed using syntax with the MANOVA procedure. For example for comparing the two y means of factor b at level 1 of factor c we run

MANOVA Y BY b(1,2) c(1,3) /DESIGN b WITHIN c(1) VS WITHIN.

The WITHIN term above is the error term from fitting the full factorial model (b, c and the interaction b x c) and is recommended as the error term in simple effects involving only between subjects factors (Boniface, 1995, p.155).

See also here.

References

Boniface, DR (1995) Experiment design and statistical methods for behavioural and social research. Chapman and Hall:London.

Field, A (2013) Discovering statistics using IBM SPSS Statistics. Fourth Edition. Sage:London.