Havnt played with one of these for ages. Screws holding the unit(s) are top corners usually. Possible a bit of the dash skin is masking seeing the screw. Dont force the instrument cluster - the plastic will be brittle and may snap. Then it is super glue etc..
Good - Fast - Cheap: Pick any two.
E23 735i; E30 318i; E32 735iL (X 2 - 1 White and 1 Light Blue); E34 535i; E38 735iL; R50 Cooper; R55 Cooper Clubman.
E21 318i; E32 735iL; E34 535i; E38 730iL; E53 4.4i