1
I meant to help you.
To express this intention, you would sign 'I', followed by the sign for 'meant', and then 'help'. Because ASL doesn't use English past-tense endings, the past tense is usually understood from the context of the conversation or by establishing the timeframe earlier.
2
What did that mean?
You can use this exact same sign for the present-tense word 'mean'. Simply sign 'THAT', 'MEAN', and then use the 'WHAT' sign. Remember to furrow your eyebrows while signing 'what' to clearly indicate that you are asking a 'wh-' question.
3
That is not what I meant.
For this sentence, you would sign 'THAT', 'NOT', 'I', 'MEANT'. When you sign 'not', be sure to shake your head slightly. Using this non-manual marker is essential in ASL to clearly show the negative meaning of the sentence.