The Application Process

I work for a small consutlancy company with maximum 15 people in total. The company is based in holland and has a branch at Indonesia. But I have been working at the client place is IBM
from the time I have been recruited and will be working in future as well. In MBA application form, I have to write about my employer details such as
-company name,
-website,
-turnover,
-number of employees.

Is it ok to mention IBM details for the above rather than the small consultancy because of IBMbrand name

Your employer is the company paying your salary. I'm sure adcoms are familiar with situations like yours whereby you're hired by Firm A but the nature of the work puts you on-site at Client X.

Then I should go with the consultancy. But will it affect my profile if I mention a company without the brand name.

Mention the consulting Firm. You can talk about IBM in your experiences or on your resume.