Many To Many Relationship (Help, please)

I have created tables x_data, x_endowment and x_endowment_nm.

x_data:
several columns, some with ids and id/name lookups, but for simplicity I am leaving pout most of it so it does not get too confusing, hopefully.

x_endowment:

CREATE TABLE x_endowment
(id int(10) NOT NULL auto_increment
,name varchar(255) NOT NULL
,PRIMARY KEY (id)
) ENGINE InnoDB DEFAULT CHARACTER SET utf8 COLLATE=utf8_general_ci;

x_endowment_nm:

CREATE TABLE x_endowment_nm
(id_data int(11) NOT NULL
,id_endowment int(11) NOT NULL
,PRIMARY KEY (id_data,id_endowment)
) ENGINE InnoDB DEFAULT CHARACTER SET utf8 COLLATE=utf8_general_ci;

In the data project pages:
x_data, x_endowment, x_endowment_nm set as type “parent/child”

In the table options for x_data:

type: n:m 
source column: id
target table: x_endowment
target column: id
relation table: x_endowment_nm

table options for x_endowment:

none

table options for x_endowment_nm:

type: 1:n 
source column: id_data
target table: x_data
target column: id

type: 1:n 
source column: id_endowment
target table: x_endowment
target column: id

type: lookup 
source column: id_data
target table: x_data
target column: id
lookup: title

type: lookup 
source column: id_endowment
target table: x_endowment
target column: id
lookup: name

What I am getting: BE-Editing of x_data with the created WP Data Access page more or less works, meaning: the relation form below the normal fields appears. However, I am not getting the available rows from x_endowment, I can only create new rows, which then get a new row in x_endowment and a new row in x_endowment_nm with their corresponding ids.

What I was thinking of getting: being able to choose from the pool of already existing x_endowment rows and assigning them to the x_data rows, whereas x_endoement_nm holds the info about those. So I have a pool of x_endowment entries which can be reused and each only exist one time, being able to assign them to multiple x_data ids – or the other way around if you will.

I am relatively new to relations with a junction table, let alone WP data Access, so could somebody please bring some light to my rather hopeless situation?! ??

Thanks a lot!