I got it working with Migx 2.5.6 and MODX 2.2.9-pl. What does your error log say?
This is my code:
[{
"formname":"OneCell"
,"formtabs": [{
"caption":"Indeling"
,"fields": [{
"field":"fake"
,"inputTVtype":"hidden"
},{
"field":"cell_2_content"
,"caption":"Content"
,"inputTVtype":"hidden"
},{
"field":"cell_3_content"
,"caption":"Content"
,"inputTVtype":"hidden"
}]
},{
"caption":"kolom #1"
,"fields": [{
"field":"cell_1_content"
,"caption":"Content"
,"inputTVtype":"richtext"
}]
}]
},{
"formname":"TwoCells"
,"formtabs":[{
"caption":"Indeling"
,"fields": [{
"field":"fake"
,"inputTVtype":"hidden"
},{
"field":"cell_3_content"
,"caption":"Content"
,"inputTVtype":"hidden"
}]
},{
"caption":"kolom #1"
,"fields":[{
"field":"cell_1_content"
,"caption":"Content"
,"inputTVtype":"richtext"
}]
},{
"caption":"kolom #2"
,"fields":[{
"field":"cell_2_content"
,"caption":"Content"
,"inputTVtype":"richtext"
}]
}]
},{
"formname":"ThreeCells"
,"formtabs":[{
"caption":"Indeling"
,"fields": [{
"field":"fake"
,"inputTVtype":"hidden"
}]
},{
"caption":"kolom #1"
,"fields": [{
"field":"cell_1_content"
,"caption":"Content"
,"inputTVtype":"richtext"
}]
},{
"caption":"kolom #2"
,"fields":[{
"field":"cell_2_content"
,"caption":"Content"
,"inputTVtype":"richtext"
}]
},{
"caption":"kolom #3"
,"fields":[{
"field":"cell_3_content"
,"caption":"Content"
,"inputTVtype":"richtext"
}]
}]
}]