Trados doesn't have that button, so I assume you're referring to the Word function of showing or hiding non-printable characters (including hidden text)?
I have a trados freelance 6.5 version and two questions.
1. I cannot hide bilingual display while translating in Trados, I tried hide/show button, but it does not work.
First of all, it is strongly recommended to keep bilingual display visible when translating - hiding the source text is ok for proofing, but even then you should never edit anything without displaying hidden text and tags.
Check your display options in Word (Tools - Options - Display - Non-printable characters) - sounds as if "hidden text" is activated (which overrides the Hide/Show button).
2. Recently, I had a Word document of about 70 pages to be translated, since I did it in bilingual display of Trados, when it reached about 80 pages, it reported to me that Word memory was insufficient, and that it cannot finish operation of closing Trados bilingual window How can I solve this?
Usually, there's nothing to solve, as Word and Trados behave normally despite the warning message (which mostly occurs in tables). To be on the safe side, I would save the document straight afterwards.