Resource Commitment in P3 version 3.1

Could anyone of you help me please on the following:

Activities:
1. Building Construction, dur:6mths, resources:Snr Project Engineer 70% for the first 3 months and Project Engineer 100% throughout
2. Road Construction, dur: 8mths, resources: Snr Road Engineer 80% for the first 2 months and Road Engineer 100% from month no 3 onwards till completion.
And so on..

Provided I’ve entered all the appropriate info above into P3 (i.e.: activity codes, durations, sequence, resources, etc):
How do I allocate that 70% of Snr Project Engineer time onto the activities? (We can do this in MS Project via ’Resource Usage’ option).

Thanks alot guys

Regards
Donny