Documents required to obtain business visa to attend ISOCARP Congress

I am a Sri Lankan Public official wishing to attend the ISOCARP Congress(the annual congress of the International Society of City and Regional Planners) to be held at Marco Polo hotel in Wuhan from 24th to 28th October. Ive the letter of invitation received from ISOCARP and the receipt confirming payment and registration for the congress.What other documents do I need to obtain visa and also how do I get transit visa for Hong Kong as I will be flying via Hong Kong on Cathay Pacific?